"re" — Regular expression operations
************************************

**Source code:** Lib/re/

======================================================================

This module provides regular expression matching operations similar to
those found in Perl.

Both patterns and strings to be searched can be Unicode strings
("str") as well as 8-bit strings ("bytes"). However, Unicode strings
and 8-bit strings cannot be mixed: that is, you cannot match a Unicode
string with a bytes pattern or vice-versa; similarly, when asking for
a substitution, the replacement string must be of the same type as
both the pattern and the search string.

Regular expressions use the backslash character ("'\'") to indicate
special forms or to allow special characters to be used without
invoking their special meaning.  This collides with Python’s usage of
the same character for the same purpose in string literals; for
example, to match a literal backslash, one might have to write
"'\\\\'" as the pattern string, because the regular expression must be
"\\", and each backslash must be expressed as "\\" inside a regular
Python string literal. Also, please note that any invalid escape
sequences in Python’s usage of the backslash in string literals now
generate a "DeprecationWarning" and in the future this will become a
"SyntaxError". This behaviour will happen even if it is a valid escape
sequence for a regular expression.

The solution is to use Python’s raw string notation for regular
expression patterns; backslashes are not handled in any special way in
a string literal prefixed with "'r'".  So "r"\n"" is a two-character
string containing "'\'" and "'n'", while ""\n"" is a one-character
string containing a newline.  Usually patterns will be expressed in
Python code using this raw string notation.

It is important to note that most regular expression operations are
available as module-level functions and methods on compiled regular
expressions.  The functions are shortcuts that don’t require you to
compile a regex object first, but miss some fine-tuning parameters.

See also:

  The third-party regex module, which has an API compatible with the
  standard library "re" module, but offers additional functionality
  and a more thorough Unicode support.


Regular Expression Syntax
=========================

A regular expression (or RE) specifies a set of strings that matches
it; the functions in this module let you check if a particular string
matches a given regular expression (or if a given regular expression
matches a particular string, which comes down to the same thing).

Regular expressions can be concatenated to form new regular
expressions; if *A* and *B* are both regular expressions, then *AB* is
also a regular expression. In general, if a string *p* matches *A* and
another string *q* matches *B*, the string *pq* will match AB.  This
holds unless *A* or *B* contain low precedence operations; boundary
conditions between *A* and *B*; or have numbered group references.
Thus, complex expressions can easily be constructed from simpler
primitive expressions like the ones described here.  For details of
the theory and implementation of regular expressions, consult the
Friedl book [Frie09], or almost any textbook about compiler
construction.

A brief explanation of the format of regular expressions follows.  For
further information and a gentler presentation, consult the Regular
Expression HOWTO.

Regular expressions can contain both special and ordinary characters.
Most ordinary characters, like "'A'", "'a'", or "'0'", are the
simplest regular expressions; they simply match themselves.  You can
concatenate ordinary characters, so "last" matches the string
"'last'".  (In the rest of this section, we’ll write RE’s in "this
special style", usually without quotes, and strings to be matched "'in
single quotes'".)

Some characters, like "'|'" or "'('", are special. Special characters
either stand for classes of ordinary characters, or affect how the
regular expressions around them are interpreted.

Repetition operators or quantifiers ("*", "+", "?", "{m,n}", etc)
cannot be directly nested. This avoids ambiguity with the non-greedy
modifier suffix "?", and with other modifiers in other
implementations. To apply a second repetition to an inner repetition,
parentheses may be used. For example, the expression "(?:a{6})*"
matches any multiple of six "'a'" characters.

The special characters are:

"."
   (Dot.)  In the default mode, this matches any character except a
   newline.  If the "DOTALL" flag has been specified, this matches any
   character including a newline.

"^"
   (Caret.)  Matches the start of the string, and in "MULTILINE" mode
   also matches immediately after each newline.

"$"
   Matches the end of the string or just before the newline at the end
   of the string, and in "MULTILINE" mode also matches before a
   newline.  "foo" matches both ‘foo’ and ‘foobar’, while the regular
   expression "foo$" matches only ‘foo’.  More interestingly,
   searching for "foo.$" in "'foo1\nfoo2\n'" matches ‘foo2’ normally,
   but ‘foo1’ in "MULTILINE" mode; searching for a single "$" in
   "'foo\n'" will find two (empty) matches: one just before the
   newline, and one at the end of the string.

"*"
   Causes the resulting RE to match 0 or more repetitions of the
   preceding RE, as many repetitions as are possible.  "ab*" will
   match ‘a’, ‘ab’, or ‘a’ followed by any number of ‘b’s.

"+"
   Causes the resulting RE to match 1 or more repetitions of the
   preceding RE. "ab+" will match ‘a’ followed by any non-zero number
   of ‘b’s; it will not match just ‘a’.

"?"
   Causes the resulting RE to match 0 or 1 repetitions of the
   preceding RE. "ab?" will match either ‘a’ or ‘ab’.

"*?", "+?", "??"
   The "'*'", "'+'", and "'?'" quantifiers are all *greedy*; they
   match as much text as possible.  Sometimes this behaviour isn’t
   desired; if the RE "<.*>" is matched against "'<a> b <c>'", it will
   match the entire string, and not just "'<a>'".  Adding "?" after
   the quantifier makes it perform the match in *non-greedy* or
   *minimal* fashion; as *few* characters as possible will be matched.
   Using the RE "<.*?>" will match only "'<a>'".

"*+", "++", "?+"
   Like the "'*'", "'+'", and "'?'" quantifiers, those where "'+'" is
   appended also match as many times as possible. However, unlike the
   true greedy quantifiers, these do not allow back-tracking when the
   expression following it fails to match. These are known as
   *possessive* quantifiers. For example, "a*a" will match "'aaaa'"
   because the "a*" will match all 4 "'a'"s, but, when the final "'a'"
   is encountered, the expression is backtracked so that in the end
   the "a*" ends up matching 3 "'a'"s total, and the fourth "'a'" is
   matched by the final "'a'". However, when "a*+a" is used to match
   "'aaaa'", the "a*+" will match all 4 "'a'", but when the final
   "'a'" fails to find any more characters to match, the expression
   cannot be backtracked and will thus fail to match. "x*+", "x++" and
   "x?+" are equivalent to "(?>x*)", "(?>x+)" and "(?>x?)"
   correspondingly.

   New in version 3.11.

"{m}"
   Specifies that exactly *m* copies of the previous RE should be
   matched; fewer matches cause the entire RE not to match.  For
   example, "a{6}" will match exactly six "'a'" characters, but not
   five.

"{m,n}"
   Causes the resulting RE to match from *m* to *n* repetitions of the
   preceding RE, attempting to match as many repetitions as possible.
   For example, "a{3,5}" will match from 3 to 5 "'a'" characters.
   Omitting *m* specifies a lower bound of zero,  and omitting *n*
   specifies an infinite upper bound.  As an example, "a{4,}b" will
   match "'aaaab'" or a thousand "'a'" characters followed by a "'b'",
   but not "'aaab'". The comma may not be omitted or the modifier
   would be confused with the previously described form.

"{m,n}?"
   Causes the resulting RE to match from *m* to *n* repetitions of the
   preceding RE, attempting to match as *few* repetitions as possible.
   This is the non-greedy version of the previous quantifier.  For
   example, on the 6-character string "'aaaaaa'", "a{3,5}" will match
   5 "'a'" characters, while "a{3,5}?" will only match 3 characters.

"{m,n}+"
   Causes the resulting RE to match from *m* to *n* repetitions of the
   preceding RE, attempting to match as many repetitions as possible
   *without* establishing any backtracking points. This is the
   possessive version of the quantifier above. For example, on the
   6-character string "'aaaaaa'", "a{3,5}+aa" attempt to match 5 "'a'"
   characters, then, requiring 2 more "'a'"s, will need more
   characters than available and thus fail, while "a{3,5}aa" will
   match with "a{3,5}" capturing 5, then 4 "'a'"s by backtracking and
   then the final 2 "'a'"s are matched by the final "aa" in the
   pattern. "x{m,n}+" is equivalent to "(?>x{m,n})".

   New in version 3.11.

"\"
   Either escapes special characters (permitting you to match
   characters like "'*'", "'?'", and so forth), or signals a special
   sequence; special sequences are discussed below.

   If you’re not using a raw string to express the pattern, remember
   that Python also uses the backslash as an escape sequence in string
   literals; if the escape sequence isn’t recognized by Python’s
   parser, the backslash and subsequent character are included in the
   resulting string.  However, if Python would recognize the resulting
   sequence, the backslash should be repeated twice.  This is
   complicated and hard to understand, so it’s highly recommended that
   you use raw strings for all but the simplest expressions.

"[]"
   Used to indicate a set of characters.  In a set:

   * Characters can be listed individually, e.g. "[amk]" will match
     "'a'", "'m'", or "'k'".

   * Ranges of characters can be indicated by giving two characters
     and separating them by a "'-'", for example "[a-z]" will match
     any lowercase ASCII letter, "[0-5][0-9]" will match all the two-
     digits numbers from "00" to "59", and "[0-9A-Fa-f]" will match
     any hexadecimal digit.  If "-" is escaped (e.g. "[a\-z]") or if
     it’s placed as the first or last character (e.g. "[-a]" or
     "[a-]"), it will match a literal "'-'".

   * Special characters lose their special meaning inside sets.  For
     example, "[(+*)]" will match any of the literal characters "'('",
     "'+'", "'*'", or "')'".

   * Character classes such as "\w" or "\S" (defined below) are also
     accepted inside a set, although the characters they match depend
     on the flags used.

   * Characters that are not within a range can be matched by
     *complementing* the set.  If the first character of the set is
     "'^'", all the characters that are *not* in the set will be
     matched.  For example, "[^5]" will match any character except
     "'5'", and "[^^]" will match any character except "'^'".  "^" has
     no special meaning if it’s not the first character in the set.

   * To match a literal "']'" inside a set, precede it with a
     backslash, or place it at the beginning of the set.  For example,
     both "[()[\]{}]" and "[]()[{}]" will match a right bracket, as
     well as left bracket, braces, and parentheses.

   * Support of nested sets and set operations as in Unicode Technical
     Standard #18 might be added in the future.  This would change the
     syntax, so to facilitate this change a "FutureWarning" will be
     raised in ambiguous cases for the time being. That includes sets
     starting with a literal "'['" or containing literal character
     sequences "'--'", "'&&'", "'~~'", and "'||'".  To avoid a warning
     escape them with a backslash.

   Changed in version 3.7: "FutureWarning" is raised if a character
   set contains constructs that will change semantically in the
   future.

"|"
   "A|B", where *A* and *B* can be arbitrary REs, creates a regular
   expression that will match either *A* or *B*.  An arbitrary number
   of REs can be separated by the "'|'" in this way.  This can be used
   inside groups (see below) as well.  As the target string is
   scanned, REs separated by "'|'" are tried from left to right. When
   one pattern completely matches, that branch is accepted. This means
   that once *A* matches, *B* will not be tested further, even if it
   would produce a longer overall match.  In other words, the "'|'"
   operator is never greedy.  To match a literal "'|'", use "\|", or
   enclose it inside a character class, as in "[|]".

"(...)"
   Matches whatever regular expression is inside the parentheses, and
   indicates the start and end of a group; the contents of a group can
   be retrieved after a match has been performed, and can be matched
   later in the string with the "\number" special sequence, described
   below.  To match the literals "'('" or "')'", use "\(" or "\)", or
   enclose them inside a character class: "[(]", "[)]".

"(?...)"
   This is an extension notation (a "'?'" following a "'('" is not
   meaningful otherwise).  The first character after the "'?'"
   determines what the meaning and further syntax of the construct is.
   Extensions usually do not create a new group; "(?P<name>...)" is
   the only exception to this rule. Following are the currently
   supported extensions.

"(?aiLmsux)"
   (One or more letters from the set "'a'", "'i'", "'L'", "'m'",
   "'s'", "'u'", "'x'".) The group matches the empty string; the
   letters set the corresponding flags for the entire regular
   expression:

   * "re.A" (ASCII-only matching)

   * "re.I" (ignore case)

   * "re.L" (locale dependent)

   * "re.M" (multi-line)

   * "re.S" (dot matches all)

   * "re.U" (Unicode matching)

   * "re.X" (verbose)

   (The flags are described in Module Contents.) This is useful if you
   wish to include the flags as part of the regular expression,
   instead of passing a *flag* argument to the "re.compile()"
   function. Flags should be used first in the expression string.

   Changed in version 3.11: This construction can only be used at the
   start of the expression.

"(?:...)"
   A non-capturing version of regular parentheses.  Matches whatever
   regular expression is inside the parentheses, but the substring
   matched by the group *cannot* be retrieved after performing a match
   or referenced later in the pattern.

"(?aiLmsux-imsx:...)"
   (Zero or more letters from the set "'a'", "'i'", "'L'", "'m'",
   "'s'", "'u'", "'x'", optionally followed by "'-'" followed by one
   or more letters from the "'i'", "'m'", "'s'", "'x'".) The letters
   set or remove the corresponding flags for the part of the
   expression:

   * "re.A" (ASCII-only matching)

   * "re.I" (ignore case)

   * "re.L" (locale dependent)

   * "re.M" (multi-line)

   * "re.S" (dot matches all)

   * "re.U" (Unicode matching)

   * "re.X" (verbose)

   (The flags are described in Module Contents.)

   The letters "'a'", "'L'" and "'u'" are mutually exclusive when used
   as inline flags, so they can’t be combined or follow "'-'".
   Instead, when one of them appears in an inline group, it overrides
   the matching mode in the enclosing group.  In Unicode patterns
   "(?a:...)" switches to ASCII-only matching, and "(?u:...)" switches
   to Unicode matching (default).  In bytes patterns "(?L:...)"
   switches to locale dependent matching, and "(?a:...)" switches to
   ASCII-only matching (default). This override is only in effect for
   the narrow inline group, and the original matching mode is restored
   outside of the group.

   New in version 3.6.

   Changed in version 3.7: The letters "'a'", "'L'" and "'u'" also can
   be used in a group.

"(?>...)"
   Attempts to match "..." as if it was a separate regular expression,
   and if successful, continues to match the rest of the pattern
   following it. If the subsequent pattern fails to match, the stack
   can only be unwound to a point *before* the "(?>...)" because once
   exited, the expression, known as an *atomic group*, has thrown away
   all stack points within itself. Thus, "(?>.*)." would never match
   anything because first the ".*" would match all characters
   possible, then, having nothing left to match, the final "." would
   fail to match. Since there are no stack points saved in the Atomic
   Group, and there is no stack point before it, the entire expression
   would thus fail to match.

   New in version 3.11.

"(?P<name>...)"
   Similar to regular parentheses, but the substring matched by the
   group is accessible via the symbolic group name *name*.  Group
   names must be valid Python identifiers, and each group name must be
   defined only once within a regular expression.  A symbolic group is
   also a numbered group, just as if the group were not named.

   Named groups can be referenced in three contexts.  If the pattern
   is "(?P<quote>['"]).*?(?P=quote)" (i.e. matching a string quoted
   with either single or double quotes):

   +-----------------------------------------+------------------------------------+
   | Context of reference to group “quote”   | Ways to reference it               |
   |=========================================|====================================|
   | in the same pattern itself              | * "(?P=quote)" (as shown)  * "\1"  |
   +-----------------------------------------+------------------------------------+
   | when processing match object *m*        | * "m.group('quote')"  *            |
   |                                         | "m.end('quote')" (etc.)            |
   +-----------------------------------------+------------------------------------+
   | in a string passed to the *repl*        | * "\g<quote>"  * "\g<1>"  * "\1"   |
   | argument of "re.sub()"                  |                                    |
   +-----------------------------------------+------------------------------------+

   Deprecated since version 3.11: Group *name* containing characters
   outside the ASCII range ("b'\x00'"-"b'\x7f'") in "bytes" patterns.

"(?P=name)"
   A backreference to a named group; it matches whatever text was
   matched by the earlier group named *name*.

"(?#...)"
   A comment; the contents of the parentheses are simply ignored.

"(?=...)"
   Matches if "..." matches next, but doesn’t consume any of the
   string.  This is called a *lookahead assertion*.  For example,
   "Isaac (?=Asimov)" will match "'Isaac '" only if it’s followed by
   "'Asimov'".

"(?!...)"
   Matches if "..." doesn’t match next.  This is a *negative lookahead
   assertion*. For example, "Isaac (?!Asimov)" will match "'Isaac '"
   only if it’s *not* followed by "'Asimov'".

"(?<=...)"
   Matches if the current position in the string is preceded by a
   match for "..." that ends at the current position.  This is called
   a *positive lookbehind assertion*. "(?<=abc)def" will find a match
   in "'abcdef'", since the lookbehind will back up 3 characters and
   check if the contained pattern matches. The contained pattern must
   only match strings of some fixed length, meaning that "abc" or
   "a|b" are allowed, but "a*" and "a{3,4}" are not.  Note that
   patterns which start with positive lookbehind assertions will not
   match at the beginning of the string being searched; you will most
   likely want to use the "search()" function rather than the
   "match()" function:

   >>> import re
   >>> m = re.search('(?<=abc)def', 'abcdef')
   >>> m.group(0)
   'def'

   This example looks for a word following a hyphen:

   >>> m = re.search(r'(?<=-)\w+', 'spam-egg')
   >>> m.group(0)
   'egg'

   Changed in version 3.5: Added support for group references of fixed
   length.

"(?<!...)"
   Matches if the current position in the string is not preceded by a
   match for "...".  This is called a *negative lookbehind assertion*.
   Similar to positive lookbehind assertions, the contained pattern
   must only match strings of some fixed length.  Patterns which start
   with negative lookbehind assertions may match at the beginning of
   the string being searched.

"(?(id/name)yes-pattern|no-pattern)"
   Will try to match with "yes-pattern" if the group with given *id*
   or *name* exists, and with "no-pattern" if it doesn’t. "no-pattern"
   is optional and can be omitted. For example,
   "(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)" is a poor email matching
   pattern, which will match with "'<user@host.com>'" as well as
   "'user@host.com'", but not with "'<user@host.com'" nor
   "'user@host.com>'".

   Deprecated since version 3.11: Group *id* containing anything
   except ASCII digits. Group *name* containing characters outside the
   ASCII range ("b'\x00'"-"b'\x7f'") in "bytes" replacement strings.

The special sequences consist of "'\'" and a character from the list
below. If the ordinary character is not an ASCII digit or an ASCII
letter, then the resulting RE will match the second character.  For
example, "\$" matches the character "'$'".

"\number"
   Matches the contents of the group of the same number.  Groups are
   numbered starting from 1.  For example, "(.+) \1" matches "'the
   the'" or "'55 55'", but not "'thethe'" (note the space after the
   group).  This special sequence can only be used to match one of the
   first 99 groups.  If the first digit of *number* is 0, or *number*
   is 3 octal digits long, it will not be interpreted as a group
   match, but as the character with octal value *number*. Inside the
   "'['" and "']'" of a character class, all numeric escapes are
   treated as characters.

"\A"
   Matches only at the start of the string.

"\b"
   Matches the empty string, but only at the beginning or end of a
   word. A word is defined as a sequence of word characters. Note that
   formally, "\b" is defined as the boundary between a "\w" and a "\W"
   character (or vice versa), or between "\w" and the beginning or end
   of the string. This means that "r'\bat\b'" matches "'at'", "'at.'",
   "'(at)'", and "'as at ay'" but not "'attempt'" or "'atlas'".

   The default word characters in Unicode (str) patterns are Unicode
   alphanumerics and the underscore, but this can be changed by using
   the "ASCII" flag. Word boundaries are determined by the current
   locale if the "LOCALE" flag is used.

   Note:

     Inside a character range, "\b" represents the backspace
     character, for compatibility with Python’s string literals.

"\B"
   Matches the empty string, but only when it is *not* at the
   beginning or end of a word. This means that "r'at\B'" matches
   "'athens'", "'atom'", "'attorney'", but not "'at'", "'at.'", or
   "'at!'". "\B" is the opposite of "\b", so word characters in
   Unicode (str) patterns are Unicode alphanumerics or the underscore,
   although this can be changed by using the "ASCII" flag. Word
   boundaries are determined by the current locale if the "LOCALE"
   flag is used.

"\d"
   For Unicode (str) patterns:
      Matches any Unicode decimal digit (that is, any character in
      Unicode character category [Nd]). This includes "[0-9]", and
      also many other digit characters.

      Matches "[0-9]" if the "ASCII" flag is used.

   For 8-bit (bytes) patterns:
      Matches any decimal digit in the ASCII character set; this is
      equivalent to "[0-9]".

"\D"
   Matches any character which is not a decimal digit. This is the
   opposite of "\d".

   Matches "[^0-9]" if the "ASCII" flag is used.

"\s"
   For Unicode (str) patterns:
      Matches Unicode whitespace characters (which includes "[
      \t\n\r\f\v]", and also many other characters, for example the
      non-breaking spaces mandated by typography rules in many
      languages).

      Matches "[ \t\n\r\f\v]" if the "ASCII" flag is used.

   For 8-bit (bytes) patterns:
      Matches characters considered whitespace in the ASCII character
      set; this is equivalent to "[ \t\n\r\f\v]".

"\S"
   Matches any character which is not a whitespace character. This is
   the opposite of "\s".

   Matches "[^ \t\n\r\f\v]" if the "ASCII" flag is used.

"\w"
   For Unicode (str) patterns:
      Matches Unicode word characters; this includes all Unicode
      alphanumeric characters (as defined by "str.isalnum()"), as well
      as the underscore ("_").

      Matches "[a-zA-Z0-9_]" if the "ASCII" flag is used.

   For 8-bit (bytes) patterns:
      Matches characters considered alphanumeric in the ASCII
      character set; this is equivalent to "[a-zA-Z0-9_]". If the
      "LOCALE" flag is used, matches characters considered
      alphanumeric in the current locale and the underscore.

"\W"
   Matches any character which is not a word character. This is the
   opposite of "\w". By default, matches non-underscore ("_")
   characters for which "str.isalnum()" returns "False".

   Matches "[^a-zA-Z0-9_]" if the "ASCII" flag is used.

   If the "LOCALE" flag is used, matches characters which are neither
   alphanumeric in the current locale nor the underscore.

"\Z"
   Matches only at the end of the string.

Most of the standard escapes supported by Python string literals are
also accepted by the regular expression parser:

   \a      \b      \f      \n
   \N      \r      \t      \u
   \U      \v      \x      \\

(Note that "\b" is used to represent word boundaries, and means
“backspace” only inside character classes.)

"'\u'", "'\U'", and "'\N'" escape sequences are only recognized in
Unicode (str) patterns. In bytes patterns they are errors. Unknown
escapes of ASCII letters are reserved for future use and treated as
errors.

Octal escapes are included in a limited form.  If the first digit is a
0, or if there are three octal digits, it is considered an octal
escape. Otherwise, it is a group reference.  As for string literals,
octal escapes are always at most three digits in length.

Changed in version 3.3: The "'\u'" and "'\U'" escape sequences have
been added.

Changed in version 3.6: Unknown escapes consisting of "'\'" and an
ASCII letter now are errors.

Changed in version 3.8: The "'\N{*name*}'" escape sequence has been
added. As in string literals, it expands to the named Unicode
character (e.g. "'\N{EM DASH}'").


Module Contents
===============

The module defines several functions, constants, and an exception.
Some of the functions are simplified versions of the full featured
methods for compiled regular expressions.  Most non-trivial
applications always use the compiled form.


Flags
-----

Changed in version 3.6: Flag constants are now instances of
"RegexFlag", which is a subclass of "enum.IntFlag".

class re.RegexFlag

   An "enum.IntFlag" class containing the regex options listed below.

   New in version 3.11: - added to "__all__"

re.A
re.ASCII

   Make "\w", "\W", "\b", "\B", "\d", "\D", "\s" and "\S" perform
   ASCII-only matching instead of full Unicode matching.  This is only
   meaningful for Unicode (str) patterns, and is ignored for bytes
   patterns.

   Corresponds to the inline flag "(?a)".

   Note:

     The "U" flag still exists for backward compatibility, but is
     redundant in Python 3 since matches are Unicode by default for
     "str" patterns, and Unicode matching isn’t allowed for bytes
     patterns. "UNICODE" and the inline flag "(?u)" are similarly
     redundant.

re.DEBUG

   Display debug information about compiled expression.

   No corresponding inline flag.

re.I
re.IGNORECASE

   Perform case-insensitive matching; expressions like "[A-Z]" will
   also  match lowercase letters. Full Unicode matching (such as "Ü"
   matching "ü") also works unless the "ASCII" flag is used to disable
   non-ASCII matches. The current locale does not change the effect of
   this flag unless the "LOCALE" flag is also used.

   Corresponds to the inline flag "(?i)".

   Note that when the Unicode patterns "[a-z]" or "[A-Z]" are used in
   combination with the "IGNORECASE" flag, they will match the 52
   ASCII letters and 4 additional non-ASCII letters: ‘İ’ (U+0130,
   Latin capital letter I with dot above), ‘ı’ (U+0131, Latin small
   letter dotless i), ‘ſ’ (U+017F, Latin small letter long s) and ‘K’
   (U+212A, Kelvin sign). If the "ASCII" flag is used, only letters
   ‘a’ to ‘z’ and ‘A’ to ‘Z’ are matched.

re.L
re.LOCALE

   Make "\w", "\W", "\b", "\B" and case-insensitive matching dependent
   on the current locale. This flag can be used only with bytes
   patterns.

   Corresponds to the inline flag "(?L)".

   Warning:

     This flag is discouraged; consider Unicode matching instead. The
     locale mechanism is very unreliable as it only handles one
     “culture” at a time and only works with 8-bit locales. Unicode
     matching is enabled by default for Unicode (str) patterns and it
     is able to handle different locales and languages.

   Changed in version 3.6: "LOCALE" can be used only with bytes
   patterns and is not compatible with "ASCII".

   Changed in version 3.7: Compiled regular expression objects with
   the "LOCALE" flag no longer depend on the locale at compile time.
   Only the locale at matching time affects the result of matching.

re.M
re.MULTILINE

   When specified, the pattern character "'^'" matches at the
   beginning of the string and at the beginning of each line
   (immediately following each newline); and the pattern character
   "'$'" matches at the end of the string and at the end of each line
   (immediately preceding each newline).  By default, "'^'" matches
   only at the beginning of the string, and "'$'" only at the end of
   the string and immediately before the newline (if any) at the end
   of the string.

   Corresponds to the inline flag "(?m)".

re.NOFLAG

   Indicates no flag being applied, the value is "0".  This flag may
   be used as a default value for a function keyword argument or as a
   base value that will be conditionally ORed with other flags.
   Example of use as a default value:

      def myfunc(text, flag=re.NOFLAG):
          return re.match(text, flag)

   New in version 3.11.

re.S
re.DOTALL

   Make the "'.'" special character match any character at all,
   including a newline; without this flag, "'.'" will match anything
   *except* a newline.

   Corresponds to the inline flag "(?s)".

re.U
re.UNICODE

   In Python 3, Unicode characters are matched by default for "str"
   patterns. This flag is therefore redundant with **no effect** and
   is only kept for backward compatibility.

   See "ASCII" to restrict matching to ASCII characters instead.

re.X
re.VERBOSE

   This flag allows you to write regular expressions that look nicer
   and are more readable by allowing you to visually separate logical
   sections of the pattern and add comments. Whitespace within the
   pattern is ignored, except when in a character class, or when
   preceded by an unescaped backslash, or within tokens like "*?",
   "(?:" or "(?P<...>". For example, "(? :" and "* ?" are not allowed.
   When a line contains a "#" that is not in a character class and is
   not preceded by an unescaped backslash, all characters from the
   leftmost such "#" through the end of the line are ignored.

   This means that the two following regular expression objects that
   match a decimal number are functionally equal:

      a = re.compile(r"""\d +  # the integral part
                         \.    # the decimal point
                         \d *  # some fractional digits""", re.X)
      b = re.compile(r"\d+\.\d*")

   Corresponds to the inline flag "(?x)".


Functions
---------

re.compile(pattern, flags=0)

   Compile a regular expression pattern into a regular expression
   object, which can be used for matching using its "match()",
   "search()" and other methods, described below.

   The expression’s behaviour can be modified by specifying a *flags*
   value. Values can be any of the flags variables, combined using
   bitwise OR (the "|" operator).

   The sequence

      prog = re.compile(pattern)
      result = prog.match(string)

   is equivalent to

      result = re.match(pattern, string)

   but using "re.compile()" and saving the resulting regular
   expression object for reuse is more efficient when the expression
   will be used several times in a single program.

   Note:

     The compiled versions of the most recent patterns passed to
     "re.compile()" and the module-level matching functions are
     cached, so programs that use only a few regular expressions at a
     time needn’t worry about compiling regular expressions.

re.search(pattern, string, flags=0)

   Scan through *string* looking for the first location where the
   regular expression *pattern* produces a match, and return a
   corresponding "Match". Return "None" if no position in the string
   matches the pattern; note that this is different from finding a
   zero-length match at some point in the string.

re.match(pattern, string, flags=0)

   If zero or more characters at the beginning of *string* match the
   regular expression *pattern*, return a corresponding "Match".
   Return "None" if the string does not match the pattern; note that
   this is different from a zero-length match.

   Note that even in "MULTILINE" mode, "re.match()" will only match at
   the beginning of the string and not at the beginning of each line.

   If you want to locate a match anywhere in *string*, use "search()"
   instead (see also search() vs. match()).

re.fullmatch(pattern, string, flags=0)

   If the whole *string* matches the regular expression *pattern*,
   return a corresponding "Match".  Return "None" if the string does
   not match the pattern; note that this is different from a zero-
   length match.

   New in version 3.4.

re.split(pattern, string, maxsplit=0, flags=0)

   Split *string* by the occurrences of *pattern*.  If capturing
   parentheses are used in *pattern*, then the text of all groups in
   the pattern are also returned as part of the resulting list. If
   *maxsplit* is nonzero, at most *maxsplit* splits occur, and the
   remainder of the string is returned as the final element of the
   list.

      >>> re.split(r'\W+', 'Words, words, words.')
      ['Words', 'words', 'words', '']
      >>> re.split(r'(\W+)', 'Words, words, words.')
      ['Words', ', ', 'words', ', ', 'words', '.', '']
      >>> re.split(r'\W+', 'Words, words, words.', 1)
      ['Words', 'words, words.']
      >>> re.split('[a-f]+', '0a3B9', flags=re.IGNORECASE)
      ['0', '3', '9']

   If there are capturing groups in the separator and it matches at
   the start of the string, the result will start with an empty
   string.  The same holds for the end of the string:

      >>> re.split(r'(\W+)', '...words, words...')
      ['', '...', 'words', ', ', 'words', '...', '']

   That way, separator components are always found at the same
   relative indices within the result list.

   Empty matches for the pattern split the string only when not
   adjacent to a previous empty match.

      >>> re.split(r'\b', 'Words, words, words.')
      ['', 'Words', ', ', 'words', ', ', 'words', '.']
      >>> re.split(r'\W*', '...words...')
      ['', '', 'w', 'o', 'r', 'd', 's', '', '']
      >>> re.split(r'(\W*)', '...words...')
      ['', '...', '', '', 'w', '', 'o', '', 'r', '', 'd', '', 's', '...', '', '', '']

   Changed in version 3.1: Added the optional flags argument.

   Changed in version 3.7: Added support of splitting on a pattern
   that could match an empty string.

re.findall(pattern, string, flags=0)

   Return all non-overlapping matches of *pattern* in *string*, as a
   list of strings or tuples.  The *string* is scanned left-to-right,
   and matches are returned in the order found.  Empty matches are
   included in the result.

   The result depends on the number of capturing groups in the
   pattern. If there are no groups, return a list of strings matching
   the whole pattern.  If there is exactly one group, return a list of
   strings matching that group.  If multiple groups are present,
   return a list of tuples of strings matching the groups.  Non-
   capturing groups do not affect the form of the result.

   >>> re.findall(r'\bf[a-z]*', 'which foot or hand fell fastest')
   ['foot', 'fell', 'fastest']
   >>> re.findall(r'(\w+)=(\d+)', 'set width=20 and height=10')
   [('width', '20'), ('height', '10')]

   Changed in version 3.7: Non-empty matches can now start just after
   a previous empty match.

re.finditer(pattern, string, flags=0)

   Return an *iterator* yielding "Match" objects over all non-
   overlapping matches for the RE *pattern* in *string*.  The *string*
   is scanned left-to-right, and matches are returned in the order
   found.  Empty matches are included in the result.

   Changed in version 3.7: Non-empty matches can now start just after
   a previous empty match.

re.sub(pattern, repl, string, count=0, flags=0)

   Return the string obtained by replacing the leftmost non-
   overlapping occurrences of *pattern* in *string* by the replacement
   *repl*.  If the pattern isn’t found, *string* is returned
   unchanged.  *repl* can be a string or a function; if it is a
   string, any backslash escapes in it are processed.  That is, "\n"
   is converted to a single newline character, "\r" is converted to a
   carriage return, and so forth.  Unknown escapes of ASCII letters
   are reserved for future use and treated as errors.  Other unknown
   escapes such as "\&" are left alone. Backreferences, such as "\6",
   are replaced with the substring matched by group 6 in the pattern.
   For example:

      >>> re.sub(r'def\s+([a-zA-Z_][a-zA-Z_0-9]*)\s*\(\s*\):',
      ...        r'static PyObject*\npy_\1(void)\n{',
      ...        'def myfunc():')
      'static PyObject*\npy_myfunc(void)\n{'

   If *repl* is a function, it is called for every non-overlapping
   occurrence of *pattern*.  The function takes a single "Match"
   argument, and returns the replacement string.  For example:

      >>> def dashrepl(matchobj):
      ...     if matchobj.group(0) == '-': return ' '
      ...     else: return '-'
      >>> re.sub('-{1,2}', dashrepl, 'pro----gram-files')
      'pro--gram files'
      >>> re.sub(r'\sAND\s', ' & ', 'Baked Beans And Spam', flags=re.IGNORECASE)
      'Baked Beans & Spam'

   The pattern may be a string or a "Pattern".

   The optional argument *count* is the maximum number of pattern
   occurrences to be replaced; *count* must be a non-negative integer.
   If omitted or zero, all occurrences will be replaced. Empty matches
   for the pattern are replaced only when not adjacent to a previous
   empty match, so "sub('x*', '-', 'abxd')" returns "'-a-b--d-'".

   In string-type *repl* arguments, in addition to the character
   escapes and backreferences described above, "\g<name>" will use the
   substring matched by the group named "name", as defined by the
   "(?P<name>...)" syntax. "\g<number>" uses the corresponding group
   number; "\g<2>" is therefore equivalent to "\2", but isn’t
   ambiguous in a replacement such as "\g<2>0".  "\20" would be
   interpreted as a reference to group 20, not a reference to group 2
   followed by the literal character "'0'".  The backreference "\g<0>"
   substitutes in the entire substring matched by the RE.

   Changed in version 3.1: Added the optional flags argument.

   Changed in version 3.5: Unmatched groups are replaced with an empty
   string.

   Changed in version 3.6: Unknown escapes in *pattern* consisting of
   "'\'" and an ASCII letter now are errors.

   Changed in version 3.7: Unknown escapes in *repl* consisting of
   "'\'" and an ASCII letter now are errors.

   Changed in version 3.7: Empty matches for the pattern are replaced
   when adjacent to a previous non-empty match.

   Deprecated since version 3.11: Group *id* containing anything
   except ASCII digits. Group *name* containing characters outside the
   ASCII range ("b'\x00'"-"b'\x7f'") in "bytes" replacement strings.

re.subn(pattern, repl, string, count=0, flags=0)

   Perform the same operation as "sub()", but return a tuple
   "(new_string, number_of_subs_made)".

   Changed in version 3.1: Added the optional flags argument.

   Changed in version 3.5: Unmatched groups are replaced with an empty
   string.

re.escape(pattern)

   Escape special characters in *pattern*. This is useful if you want
   to match an arbitrary literal string that may have regular
   expression metacharacters in it.  For example:

      >>> print(re.escape('https://www.python.org'))
      https://www\.python\.org

      >>> legal_chars = string.ascii_lowercase + string.digits + "!#$%&'*+-.^_`|~:"
      >>> print('[%s]+' % re.escape(legal_chars))
      [abcdefghijklmnopqrstuvwxyz0123456789!\#\$%\&'\*\+\-\.\^_`\|\~:]+

      >>> operators = ['+', '-', '*', '/', '**']
      >>> print('|'.join(map(re.escape, sorted(operators, reverse=True))))
      /|\-|\+|\*\*|\*

   This function must not be used for the replacement string in
   "sub()" and "subn()", only backslashes should be escaped.  For
   example:

      >>> digits_re = r'\d+'
      >>> sample = '/usr/sbin/sendmail - 0 errors, 12 warnings'
      >>> print(re.sub(digits_re, digits_re.replace('\\', r'\\'), sample))
      /usr/sbin/sendmail - \d+ errors, \d+ warnings

   Changed in version 3.3: The "'_'" character is no longer escaped.

   Changed in version 3.7: Only characters that can have special
   meaning in a regular expression are escaped. As a result, "'!'",
   "'"'", "'%'", ""'"", "','", "'/'", "':'", "';'", "'<'", "'='",
   "'>'", "'@'", and ""`"" are no longer escaped.

re.purge()

   Clear the regular expression cache.


Exceptions
----------

exception re.error(msg, pattern=None, pos=None)

   Exception raised when a string passed to one of the functions here
   is not a valid regular expression (for example, it might contain
   unmatched parentheses) or when some other error occurs during
   compilation or matching.  It is never an error if a string contains
   no match for a pattern.  The error instance has the following
   additional attributes:

   msg

      The unformatted error message.

   pattern

      The regular expression pattern.

   pos

      The index in *pattern* where compilation failed (may be "None").

   lineno

      The line corresponding to *pos* (may be "None").

   colno

      The column corresponding to *pos* (may be "None").

   Changed in version 3.5: Added additional attributes.


Regular Expression Objects
==========================

class re.Pattern

   Compiled regular expression object returned by "re.compile()".

   Changed in version 3.9: "re.Pattern" supports "[]" to indicate a
   Unicode (str) or bytes pattern. See Generic Alias Type.

Pattern.search(string[, pos[, endpos]])

   Scan through *string* looking for the first location where this
   regular expression produces a match, and return a corresponding
   "Match". Return "None" if no position in the string matches the
   pattern; note that this is different from finding a zero-length
   match at some point in the string.

   The optional second parameter *pos* gives an index in the string
   where the search is to start; it defaults to "0".  This is not
   completely equivalent to slicing the string; the "'^'" pattern
   character matches at the real beginning of the string and at
   positions just after a newline, but not necessarily at the index
   where the search is to start.

   The optional parameter *endpos* limits how far the string will be
   searched; it will be as if the string is *endpos* characters long,
   so only the characters from *pos* to "endpos - 1" will be searched
   for a match.  If *endpos* is less than *pos*, no match will be
   found; otherwise, if *rx* is a compiled regular expression object,
   "rx.search(string, 0, 50)" is equivalent to "rx.search(string[:50],
   0)".

      >>> pattern = re.compile("d")
      >>> pattern.search("dog")     # Match at index 0
      <re.Match object; span=(0, 1), match='d'>
      >>> pattern.search("dog", 1)  # No match; search doesn't include the "d"

Pattern.match(string[, pos[, endpos]])

   If zero or more characters at the *beginning* of *string* match
   this regular expression, return a corresponding "Match". Return
   "None" if the string does not match the pattern; note that this is
   different from a zero-length match.

   The optional *pos* and *endpos* parameters have the same meaning as
   for the "search()" method.

      >>> pattern = re.compile("o")
      >>> pattern.match("dog")      # No match as "o" is not at the start of "dog".
      >>> pattern.match("dog", 1)   # Match as "o" is the 2nd character of "dog".
      <re.Match object; span=(1, 2), match='o'>

   If you want to locate a match anywhere in *string*, use "search()"
   instead (see also search() vs. match()).

Pattern.fullmatch(string[, pos[, endpos]])

   If the whole *string* matches this regular expression, return a
   corresponding "Match".  Return "None" if the string does not match
   the pattern; note that this is different from a zero-length match.

   The optional *pos* and *endpos* parameters have the same meaning as
   for the "search()" method.

      >>> pattern = re.compile("o[gh]")
      >>> pattern.fullmatch("dog")      # No match as "o" is not at the start of "dog".
      >>> pattern.fullmatch("ogre")     # No match as not the full string matches.
      >>> pattern.fullmatch("doggie", 1, 3)   # Matches within given limits.
      <re.Match object; span=(1, 3), match='og'>

   New in version 3.4.

Pattern.split(string, maxsplit=0)

   Identical to the "split()" function, using the compiled pattern.

Pattern.findall(string[, pos[, endpos]])

   Similar to the "findall()" function, using the compiled pattern,
   but also accepts optional *pos* and *endpos* parameters that limit
   the search region like for "search()".

Pattern.finditer(string[, pos[, endpos]])

   Similar to the "finditer()" function, using the compiled pattern,
   but also accepts optional *pos* and *endpos* parameters that limit
   the search region like for "search()".

Pattern.sub(repl, string, count=0)

   Identical to the "sub()" function, using the compiled pattern.

Pattern.subn(repl, string, count=0)

   Identical to the "subn()" function, using the compiled pattern.

Pattern.flags

   The regex matching flags.  This is a combination of the flags given
   to "compile()", any "(?...)" inline flags in the pattern, and
   implicit flags such as "UNICODE" if the pattern is a Unicode
   string.

Pattern.groups

   The number of capturing groups in the pattern.

Pattern.groupindex

   A dictionary mapping any symbolic group names defined by "(?P<id>)"
   to group numbers.  The dictionary is empty if no symbolic groups
   were used in the pattern.

Pattern.pattern

   The pattern string from which the pattern object was compiled.

Changed in version 3.7: Added support of "copy.copy()" and
"copy.deepcopy()".  Compiled regular expression objects are considered
atomic.


Match Objects
=============

Match objects always have a boolean value of "True". Since "match()"
and "search()" return "None" when there is no match, you can test
whether there was a match with a simple "if" statement:

   match = re.search(pattern, string)
   if match:
       process(match)

class re.Match

   Match object returned by successful "match"es and "search"es.

   Changed in version 3.9: "re.Match" supports "[]" to indicate a
   Unicode (str) or bytes match. See Generic Alias Type.

Match.expand(template)

   Return the string obtained by doing backslash substitution on the
   template string *template*, as done by the "sub()" method. Escapes
   such as "\n" are converted to the appropriate characters, and
   numeric backreferences ("\1", "\2") and named backreferences
   ("\g<1>", "\g<name>") are replaced by the contents of the
   corresponding group. The backreference "\g<0>" will be replaced by
   the entire match.

   Changed in version 3.5: Unmatched groups are replaced with an empty
   string.

Match.group([group1, ...])

   Returns one or more subgroups of the match.  If there is a single
   argument, the result is a single string; if there are multiple
   arguments, the result is a tuple with one item per argument.
   Without arguments, *group1* defaults to zero (the whole match is
   returned). If a *groupN* argument is zero, the corresponding return
   value is the entire matching string; if it is in the inclusive
   range [1..99], it is the string matching the corresponding
   parenthesized group.  If a group number is negative or larger than
   the number of groups defined in the pattern, an "IndexError"
   exception is raised. If a group is contained in a part of the
   pattern that did not match, the corresponding result is "None". If
   a group is contained in a part of the pattern that matched multiple
   times, the last match is returned.

      >>> m = re.match(r"(\w+) (\w+)", "Isaac Newton, physicist")
      >>> m.group(0)       # The entire match
      'Isaac Newton'
      >>> m.group(1)       # The first parenthesized subgroup.
      'Isaac'
      >>> m.group(2)       # The second parenthesized subgroup.
      'Newton'
      >>> m.group(1, 2)    # Multiple arguments give us a tuple.
      ('Isaac', 'Newton')

   If the regular expression uses the "(?P<name>...)" syntax, the
   *groupN* arguments may also be strings identifying groups by their
   group name.  If a string argument is not used as a group name in
   the pattern, an "IndexError" exception is raised.

   A moderately complicated example:

      >>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\w+)", "Malcolm Reynolds")
      >>> m.group('first_name')
      'Malcolm'
      >>> m.group('last_name')
      'Reynolds'

   Named groups can also be referred to by their index:

      >>> m.group(1)
      'Malcolm'
      >>> m.group(2)
      'Reynolds'

   If a group matches multiple times, only the last match is
   accessible:

      >>> m = re.match(r"(..)+", "a1b2c3")  # Matches 3 times.
      >>> m.group(1)                        # Returns only the last match.
      'c3'

Match.__getitem__(g)

   This is identical to "m.group(g)".  This allows easier access to an
   individual group from a match:

      >>> m = re.match(r"(\w+) (\w+)", "Isaac Newton, physicist")
      >>> m[0]       # The entire match
      'Isaac Newton'
      >>> m[1]       # The first parenthesized subgroup.
      'Isaac'
      >>> m[2]       # The second parenthesized subgroup.
      'Newton'

   Named groups are supported as well:

      >>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\w+)", "Isaac Newton")
      >>> m['first_name']
      'Isaac'
      >>> m['last_name']
      'Newton'

   New in version 3.6.

Match.groups(default=None)

   Return a tuple containing all the subgroups of the match, from 1 up
   to however many groups are in the pattern.  The *default* argument
   is used for groups that did not participate in the match; it
   defaults to "None".

   For example:

      >>> m = re.match(r"(\d+)\.(\d+)", "24.1632")
      >>> m.groups()
      ('24', '1632')

   If we make the decimal place and everything after it optional, not
   all groups might participate in the match.  These groups will
   default to "None" unless the *default* argument is given:

      >>> m = re.match(r"(\d+)\.?(\d+)?", "24")
      >>> m.groups()      # Second group defaults to None.
      ('24', None)
      >>> m.groups('0')   # Now, the second group defaults to '0'.
      ('24', '0')

Match.groupdict(default=None)

   Return a dictionary containing all the *named* subgroups of the
   match, keyed by the subgroup name.  The *default* argument is used
   for groups that did not participate in the match; it defaults to
   "None".  For example:

      >>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\w+)", "Malcolm Reynolds")
      >>> m.groupdict()
      {'first_name': 'Malcolm', 'last_name': 'Reynolds'}

Match.start([group])
Match.end([group])

   Return the indices of the start and end of the substring matched by
   *group*; *group* defaults to zero (meaning the whole matched
   substring). Return "-1" if *group* exists but did not contribute to
   the match.  For a match object *m*, and a group *g* that did
   contribute to the match, the substring matched by group *g*
   (equivalent to "m.group(g)") is

      m.string[m.start(g):m.end(g)]

   Note that "m.start(group)" will equal "m.end(group)" if *group*
   matched a null string.  For example, after "m = re.search('b(c?)',
   'cba')", "m.start(0)" is 1, "m.end(0)" is 2, "m.start(1)" and
   "m.end(1)" are both 2, and "m.start(2)" raises an "IndexError"
   exception.

   An example that will remove *remove_this* from email addresses:

      >>> email = "tony@tiremove_thisger.net"
      >>> m = re.search("remove_this", email)
      >>> email[:m.start()] + email[m.end():]
      'tony@tiger.net'

Match.span([group])

   For a match *m*, return the 2-tuple "(m.start(group),
   m.end(group))". Note that if *group* did not contribute to the
   match, this is "(-1, -1)". *group* defaults to zero, the entire
   match.

Match.pos

   The value of *pos* which was passed to the "search()" or "match()"
   method of a regex object.  This is the index into the string at
   which the RE engine started looking for a match.

Match.endpos

   The value of *endpos* which was passed to the "search()" or
   "match()" method of a regex object.  This is the index into the
   string beyond which the RE engine will not go.

Match.lastindex

   The integer index of the last matched capturing group, or "None" if
   no group was matched at all. For example, the expressions "(a)b",
   "((a)(b))", and "((ab))" will have "lastindex == 1" if applied to
   the string "'ab'", while the expression "(a)(b)" will have
   "lastindex == 2", if applied to the same string.

Match.lastgroup

   The name of the last matched capturing group, or "None" if the
   group didn’t have a name, or if no group was matched at all.

Match.re

   The regular expression object whose "match()" or "search()" method
   produced this match instance.

Match.string

   The string passed to "match()" or "search()".

Changed in version 3.7: Added support of "copy.copy()" and
"copy.deepcopy()".  Match objects are considered atomic.


Regular Expression Examples
===========================


Checking for a Pair
-------------------

In this example, we’ll use the following helper function to display
match objects a little more gracefully:

   def displaymatch(match):
       if match is None:
           return None
       return '<Match: %r, groups=%r>' % (match.group(), match.groups())

Suppose you are writing a poker program where a player’s hand is
represented as a 5-character string with each character representing a
card, “a” for ace, “k” for king, “q” for queen, “j” for jack, “t” for
10, and “2” through “9” representing the card with that value.

To see if a given string is a valid hand, one could do the following:

   >>> valid = re.compile(r"^[a2-9tjqk]{5}$")
   >>> displaymatch(valid.match("akt5q"))  # Valid.
   "<Match: 'akt5q', groups=()>"
   >>> displaymatch(valid.match("akt5e"))  # Invalid.
   >>> displaymatch(valid.match("akt"))    # Invalid.
   >>> displaymatch(valid.match("727ak"))  # Valid.
   "<Match: '727ak', groups=()>"

That last hand, ""727ak"", contained a pair, or two of the same valued
cards. To match this with a regular expression, one could use
backreferences as such:

   >>> pair = re.compile(r".*(.).*\1")
   >>> displaymatch(pair.match("717ak"))     # Pair of 7s.
   "<Match: '717', groups=('7',)>"
   >>> displaymatch(pair.match("718ak"))     # No pairs.
   >>> displaymatch(pair.match("354aa"))     # Pair of aces.
   "<Match: '354aa', groups=('a',)>"

To find out what card the pair consists of, one could use the
"group()" method of the match object in the following manner:

   >>> pair = re.compile(r".*(.).*\1")
   >>> pair.match("717ak").group(1)
   '7'

   # Error because re.match() returns None, which doesn't have a group() method:
   >>> pair.match("718ak").group(1)
   Traceback (most recent call last):
     File "<pyshell#23>", line 1, in <module>
       re.match(r".*(.).*\1", "718ak").group(1)
   AttributeError: 'NoneType' object has no attribute 'group'

   >>> pair.match("354aa").group(1)
   'a'


Simulating scanf()
------------------

Python does not currently have an equivalent to "scanf()".  Regular
expressions are generally more powerful, though also more verbose,
than "scanf()" format strings.  The table below offers some more-or-
less equivalent mappings between "scanf()" format tokens and regular
expressions.

+----------------------------------+-----------------------------------------------+
| "scanf()" Token                  | Regular Expression                            |
|==================================|===============================================|
| "%c"                             | "."                                           |
+----------------------------------+-----------------------------------------------+
| "%5c"                            | ".{5}"                                        |
+----------------------------------+-----------------------------------------------+
| "%d"                             | "[-+]?\d+"                                    |
+----------------------------------+-----------------------------------------------+
| "%e", "%E", "%f", "%g"           | "[-+]?(\d+(\.\d*)?|\.\d+)([eE][-+]?\d+)?"     |
+----------------------------------+-----------------------------------------------+
| "%i"                             | "[-+]?(0[xX][\dA-Fa-f]+|0[0-7]*|\d+)"         |
+----------------------------------+-----------------------------------------------+
| "%o"                             | "[-+]?[0-7]+"                                 |
+----------------------------------+-----------------------------------------------+
| "%s"                             | "\S+"                                         |
+----------------------------------+-----------------------------------------------+
| "%u"                             | "\d+"                                         |
+----------------------------------+-----------------------------------------------+
| "%x", "%X"                       | "[-+]?(0[xX])?[\dA-Fa-f]+"                    |
+----------------------------------+-----------------------------------------------+

To extract the filename and numbers from a string like

   /usr/sbin/sendmail - 0 errors, 4 warnings

you would use a "scanf()" format like

   %s - %d errors, %d warnings

The equivalent regular expression would be

   (\S+) - (\d+) errors, (\d+) warnings


search() vs. match()
--------------------

Python offers different primitive operations based on regular
expressions:

* "re.match()" checks for a match only at the beginning of the string

* "re.search()" checks for a match anywhere in the string (this is
  what Perl does by default)

* "re.fullmatch()" checks for entire string to be a match

For example:

   >>> re.match("c", "abcdef")    # No match
   >>> re.search("c", "abcdef")   # Match
   <re.Match object; span=(2, 3), match='c'>
   >>> re.fullmatch("p.*n", "python") # Match
   <re.Match object; span=(0, 6), match='python'>
   >>> re.fullmatch("r.*n", "python") # No match

Regular expressions beginning with "'^'" can be used with "search()"
to restrict the match at the beginning of the string:

   >>> re.match("c", "abcdef")    # No match
   >>> re.search("^c", "abcdef")  # No match
   >>> re.search("^a", "abcdef")  # Match
   <re.Match object; span=(0, 1), match='a'>

Note however that in "MULTILINE" mode "match()" only matches at the
beginning of the string, whereas using "search()" with a regular
expression beginning with "'^'" will match at the beginning of each
line.

   >>> re.match("X", "A\nB\nX", re.MULTILINE)  # No match
   >>> re.search("^X", "A\nB\nX", re.MULTILINE)  # Match
   <re.Match object; span=(4, 5), match='X'>


Making a Phonebook
------------------

"split()" splits a string into a list delimited by the passed pattern.
The method is invaluable for converting textual data into data
structures that can be easily read and modified by Python as
demonstrated in the following example that creates a phonebook.

First, here is the input.  Normally it may come from a file, here we
are using triple-quoted string syntax

   >>> text = """Ross McFluff: 834.345.1254 155 Elm Street
   ...
   ... Ronald Heathmore: 892.345.3428 436 Finley Avenue
   ... Frank Burger: 925.541.7625 662 South Dogwood Way
   ...
   ...
   ... Heather Albrecht: 548.326.4584 919 Park Place"""

The entries are separated by one or more newlines. Now we convert the
string into a list with each nonempty line having its own entry:

   >>> entries = re.split("\n+", text)
   >>> entries
   ['Ross McFluff: 834.345.1254 155 Elm Street',
   'Ronald Heathmore: 892.345.3428 436 Finley Avenue',
   'Frank Burger: 925.541.7625 662 South Dogwood Way',
   'Heather Albrecht: 548.326.4584 919 Park Place']

Finally, split each entry into a list with first name, last name,
telephone number, and address.  We use the "maxsplit" parameter of
"split()" because the address has spaces, our splitting pattern, in
it:

   >>> [re.split(":? ", entry, 3) for entry in entries]
   [['Ross', 'McFluff', '834.345.1254', '155 Elm Street'],
   ['Ronald', 'Heathmore', '892.345.3428', '436 Finley Avenue'],
   ['Frank', 'Burger', '925.541.7625', '662 South Dogwood Way'],
   ['Heather', 'Albrecht', '548.326.4584', '919 Park Place']]

The ":?" pattern matches the colon after the last name, so that it
does not occur in the result list.  With a "maxsplit" of "4", we could
separate the house number from the street name:

   >>> [re.split(":? ", entry, 4) for entry in entries]
   [['Ross', 'McFluff', '834.345.1254', '155', 'Elm Street'],
   ['Ronald', 'Heathmore', '892.345.3428', '436', 'Finley Avenue'],
   ['Frank', 'Burger', '925.541.7625', '662', 'South Dogwood Way'],
   ['Heather', 'Albrecht', '548.326.4584', '919', 'Park Place']]


Text Munging
------------

"sub()" replaces every occurrence of a pattern with a string or the
result of a function.  This example demonstrates using "sub()" with a
function to “munge” text, or randomize the order of all the characters
in each word of a sentence except for the first and last characters:

   >>> def repl(m):
   ...     inner_word = list(m.group(2))
   ...     random.shuffle(inner_word)
   ...     return m.group(1) + "".join(inner_word) + m.group(3)
   >>> text = "Professor Abdolmalek, please report your absences promptly."
   >>> re.sub(r"(\w)(\w+)(\w)", repl, text)
   'Poefsrosr Aealmlobdk, pslaee reorpt your abnseces plmrptoy.'
   >>> re.sub(r"(\w)(\w+)(\w)", repl, text)
   'Pofsroser Aodlambelk, plasee reoprt yuor asnebces potlmrpy.'


Finding all Adverbs
-------------------

"findall()" matches *all* occurrences of a pattern, not just the first
one as "search()" does.  For example, if a writer wanted to find all
of the adverbs in some text, they might use "findall()" in the
following manner:

   >>> text = "He was carefully disguised but captured quickly by police."
   >>> re.findall(r"\w+ly\b", text)
   ['carefully', 'quickly']


Finding all Adverbs and their Positions
---------------------------------------

If one wants more information about all matches of a pattern than the
matched text, "finditer()" is useful as it provides "Match" objects
instead of strings.  Continuing with the previous example, if a writer
wanted to find all of the adverbs *and their positions* in some text,
they would use "finditer()" in the following manner:

   >>> text = "He was carefully disguised but captured quickly by police."
   >>> for m in re.finditer(r"\w+ly\b", text):
   ...     print('%02d-%02d: %s' % (m.start(), m.end(), m.group(0)))
   07-16: carefully
   40-47: quickly


Raw String Notation
-------------------

Raw string notation ("r"text"") keeps regular expressions sane.
Without it, every backslash ("'\'") in a regular expression would have
to be prefixed with another one to escape it.  For example, the two
following lines of code are functionally identical:

   >>> re.match(r"\W(.)\1\W", " ff ")
   <re.Match object; span=(0, 4), match=' ff '>
   >>> re.match("\\W(.)\\1\\W", " ff ")
   <re.Match object; span=(0, 4), match=' ff '>

When one wants to match a literal backslash, it must be escaped in the
regular expression.  With raw string notation, this means "r"\\"".
Without raw string notation, one must use ""\\\\"", making the
following lines of code functionally identical:

   >>> re.match(r"\\", r"\\")
   <re.Match object; span=(0, 1), match='\\'>
   >>> re.match("\\\\", r"\\")
   <re.Match object; span=(0, 1), match='\\'>


Writing a Tokenizer
-------------------

A tokenizer or scanner analyzes a string to categorize groups of
characters.  This is a useful first step in writing a compiler or
interpreter.

The text categories are specified with regular expressions.  The
technique is to combine those into a single master regular expression
and to loop over successive matches:

   from typing import NamedTuple
   import re

   class Token(NamedTuple):
       type: str
       value: str
       line: int
       column: int

   def tokenize(code):
       keywords = {'IF', 'THEN', 'ENDIF', 'FOR', 'NEXT', 'GOSUB', 'RETURN'}
       token_specification = [
           ('NUMBER',   r'\d+(\.\d*)?'),  # Integer or decimal number
           ('ASSIGN',   r':='),           # Assignment operator
           ('END',      r';'),            # Statement terminator
           ('ID',       r'[A-Za-z]+'),    # Identifiers
           ('OP',       r'[+\-*/]'),      # Arithmetic operators
           ('NEWLINE',  r'\n'),           # Line endings
           ('SKIP',     r'[ \t]+'),       # Skip over spaces and tabs
           ('MISMATCH', r'.'),            # Any other character
       ]
       tok_regex = '|'.join('(?P<%s>%s)' % pair for pair in token_specification)
       line_num = 1
       line_start = 0
       for mo in re.finditer(tok_regex, code):
           kind = mo.lastgroup
           value = mo.group()
           column = mo.start() - line_start
           if kind == 'NUMBER':
               value = float(value) if '.' in value else int(value)
           elif kind == 'ID' and value in keywords:
               kind = value
           elif kind == 'NEWLINE':
               line_start = mo.end()
               line_num += 1
               continue
           elif kind == 'SKIP':
               continue
           elif kind == 'MISMATCH':
               raise RuntimeError(f'{value!r} unexpected on line {line_num}')
           yield Token(kind, value, line_num, column)

   statements = '''
       IF quantity THEN
           total := total + price * quantity;
           tax := price * 0.05;
       ENDIF;
   '''

   for token in tokenize(statements):
       print(token)

The tokenizer produces the following output:

   Token(type='IF', value='IF', line=2, column=4)
   Token(type='ID', value='quantity', line=2, column=7)
   Token(type='THEN', value='THEN', line=2, column=16)
   Token(type='ID', value='total', line=3, column=8)
   Token(type='ASSIGN', value=':=', line=3, column=14)
   Token(type='ID', value='total', line=3, column=17)
   Token(type='OP', value='+', line=3, column=23)
   Token(type='ID', value='price', line=3, column=25)
   Token(type='OP', value='*', line=3, column=31)
   Token(type='ID', value='quantity', line=3, column=33)
   Token(type='END', value=';', line=3, column=41)
   Token(type='ID', value='tax', line=4, column=8)
   Token(type='ASSIGN', value=':=', line=4, column=12)
   Token(type='ID', value='price', line=4, column=15)
   Token(type='OP', value='*', line=4, column=21)
   Token(type='NUMBER', value=0.05, line=4, column=23)
   Token(type='END', value=';', line=4, column=27)
   Token(type='ENDIF', value='ENDIF', line=5, column=4)
   Token(type='END', value=';', line=5, column=9)

[Frie09] Friedl, Jeffrey. Mastering Regular Expressions. 3rd ed.,
         O’Reilly Media, 2009. The third edition of the book no longer
         covers Python at all, but the first edition covered writing
         good regular expression patterns in great detail.
