Regex error python

re.sub(r"([^sw])(s*1)+","\1","...")

raise error, v # invalid expression
sre_constants.error: nothing to repeat

5

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.

6.2.1. 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 referenced
above, 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 qualifiers (*, +, ?, {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 'foo1nfoo2n'
matches ‘foo2’ normally, but ‘foo1’ in MULTILINE mode; searching for
a single $ in 'foon' 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 '?' qualifiers 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 qualifier 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>'.

{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 qualifier. 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.

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 depends on whether
  ASCII or LOCALE mode is in force.
• 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 both match a parenthesis.

|

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: 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), and re.X (verbose),
for the entire regular expression.
(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.

(?:...)

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:
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),
and re.X (verbose), for the part of the expression.
(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 byte pattern (?L:...) switches to locale depending
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.

(?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 argument of re.sub()	g<quote> g<1> 1

(?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('(?<=-)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>'.

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/end of the string.
This means that r'bfoob' matches 'foo', 'foo.', '(foo)',
'bar foo baz' but not 'foobar' or 'foo3'.

By default Unicode alphanumerics are the ones used in Unicode patterns, but
this can be changed by using the ASCII flag. Word boundaries are
determined by the current locale if the LOCALE flag is used.
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'pyB' matches 'python', 'py3',
'py2', but not 'py', 'py.', or 'py!'.
B is just the opposite of b, so word characters in Unicode
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. If the ASCII flag is
used only [0-9] is matched.

For 8-bit (bytes) patterns:

Matches any decimal digit; this is equivalent to [0-9].

D

Matches any character which is not a decimal digit. This is
the opposite of d. If the ASCII flag is used this
becomes the equivalent of [^0-9].

s

For Unicode (str) patterns:

Matches Unicode whitespace characters (which includes
[ tnrfv], and also many other characters, for example the
non-breaking spaces mandated by typography rules in many
languages). If the ASCII flag is used, only
[ tnrfv] is matched.

For 8-bit (bytes) patterns:

Matches characters considered whitespace in the ASCII character set;
this is equivalent to [ tnrfv].

S

Matches any character which is not a whitespace character. This is
the opposite of s. If the ASCII flag is used this
becomes the equivalent of [^ tnrfv].

w

For Unicode (str) patterns:

Matches Unicode word characters; this includes most characters
that can be part of a word in any language, as well as numbers and
the underscore. If the ASCII flag is used, only
[a-zA-Z0-9_] is matched.

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. If the ASCII flag is used this
becomes the equivalent of [^a-zA-Z0-9_]. If the LOCALE flag is
used, matches characters considered alphanumeric in the current locale
and 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
r      t      u      U
v      x      \

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

'u' and 'U' escape sequences are only recognized in Unicode
patterns. In bytes patterns they are 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.

6.2.2. 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.

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

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 following 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.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 patterns, and is ignored for byte patterns.
Corresponds to the inline flag (?a).

Note that for backward compatibility, the re.U flag still
exists (as well as its synonym re.UNICODE and its embedded
counterpart (?u)), but these are redundant in Python 3 since
matches are Unicode by default for strings (and Unicode matching
isn’t allowed for bytes).

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 re.ASCII flag is used to disable
non-ASCII matches. The current locale does not change the effect of this
flag unless the re.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. The use of this flag is discouraged as the locale mechanism
is very unreliable, it only handles one “culture” at a time, and it only
works with 8-bit locales. Unicode matching is already enabled by default
in Python 3 for Unicode (str) patterns, and it is able to handle different
locales/languages.
Corresponds to the inline flag (?L).

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

Changed in version 3.7: Compiled regular expression objects with the re.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.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.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.
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).

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 object. 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 object. 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 object. 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('W+', 'Words, words, words.')
['Words', 'words', 'words', '']
>>> re.split('(W+)', 'Words, words, words.')
['Words', ', ', 'words', ', ', 'words', '.', '']
>>> re.split('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('(W+)', '...words, words...')
['', '...', 'words', ', ', 'words', '...', '']

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

Note

split() doesn’t currently split a string on an empty pattern match.
For example:

>>> re.split('x*', 'axbc')
['a', 'bc']

Even though 'x*' also matches 0 ‘x’ before ‘a’, between ‘b’ and ‘c’,
and after ‘c’, currently these matches are ignored. The correct behavior
(i.e. splitting on empty matches too and returning ['', 'a', 'b', 'c',
'']) will be implemented in future versions of Python, but since this
is a backward incompatible change, a FutureWarning will be raised
in the meanwhile.

Patterns that can only match empty strings currently never split the
string. Since this doesn’t match the expected behavior, a
ValueError will be raised starting from Python 3.5:

>>> re.split("^$", "foonnbarn", flags=re.M)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  ...
ValueError: split() requires a non-empty pattern match.

Changed in version 3.1: Added the optional flags argument.

Changed in version 3.5: Splitting on a pattern that could match an empty string now raises
a warning. Patterns that can only match empty strings are now rejected.

re.findall(pattern, string, flags=0)¶

Return all non-overlapping matches of pattern in string, as a list of
strings. The string is scanned left-to-right, and matches are returned in
the order found. If one or more groups are present in the pattern, return a
list of groups; this will be a list of tuples if the pattern has more than
one group. Empty matches are included in the result unless they touch the
beginning of another 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 unless they touch the beginning of another
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 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'defs+([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 object
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'sANDs', ' & ', 'Baked Beans And Spam', flags=re.IGNORECASE)
'Baked Beans & Spam'

The pattern may be a string or a pattern object.

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 match, so sub('x*', '-', 'abc') returns
'-a-b-c-'.

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.

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('python.exe'))
python.exe

>>> 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 functions 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.

re.purge()¶

Clear the regular expression cache.

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.

6.2.3. Regular Expression Objects¶

Compiled regular expression objects support the following methods and
attributes:

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 object. 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 object.
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 object. 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.

6.2.4. 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)

Match objects support the following methods and attributes:

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.

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'

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.

6.2.5. Regular Expression Examples¶

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

>>> 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=()>"

>>> 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',)>"

>>> 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'

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

%s - %d errors, %d warnings

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

>>> re.match("c", "abcdef")    # No match
>>> re.search("c", "abcdef")   # Match
<re.Match object; span=(2, 3), match='c'>

>>> 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'>

>>> re.match('X', 'AnBnX', re.MULTILINE)  # No match
>>> re.search('^X', 'AnBnX', re.MULTILINE)  # Match
<re.Match object; span=(4, 5), match='X'>

>>> 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"""

>>> 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']

>>> [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']]

>>> [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']]

>>> 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.'

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

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

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

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

import collections
import re

Token = collections.namedtuple('Token', ['typ', 'value', 'line', 'column'])

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(kind)
        if kind == 'NEWLINE':
            line_start = mo.end()
            line_num += 1
        elif kind == 'SKIP':
            pass
        elif kind == 'MISMATCH':
            raise RuntimeError(f'{value!r} unexpected on line {line_num}')
        else:
            if kind == 'ID' and value in keywords:
                kind = value
            column = mo.start() - line_start
            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)

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

Так как символ в питоновских строках также необходимо экранировать, то в результате в шаблонах могут возникать конструкции вида '\\par'. Первый слеш означает, что следующий за ним символ нужно оставить «как есть». Третий также. В результате с точки зрения питона '\\' означает просто два слеша \. Теперь с точки зрения движка регулярных выражений, первый слеш экранирует второй. Тем самым как шаблон для регулярки '\\par' означает просто текст par. Для того, чтобы не было таких нагромождений слешей, перед открывающей кавычкой нужно поставить символ r, что скажет питону «не рассматривай как экранирующий символ (кроме случаев экранирования открывающей кавычки)». Соответственно можно будет писать r'\par'.

Для написания и тестирования регулярных выражений удобно использовать сервис https://regex101.com (не забудьте поставить галочку Python в разделе FLAVOR слева) или текстовый редактор Sublime text 3.

Задача 01. Регистрационные знаки транспортных средств

Задача 02. Количество слов

Задача 03. Поиск e-mailов

Ситуация становится гораздо сложнее, когда количество групп заранее не зафиксировано.
Чтобы разрешить эту проблему в синтаксисе регулярных выражений есть группировка (?:...). Можно писать круглые скобки и без значков ?:, однако от этого у группировки значительно меняется смысл, регулярка начинает работать гораздо медленнее. Об этом будет написано ниже. Итак, если REGEXP — шаблон, то (?:REGEXP) — эквивалентный ему шаблон. Разница только в том, что теперь к (?:REGEXP) можно применять квантификаторы, указывая, сколько именно раз должна повториться группа. Например, шаблон для поиска MAC-адреса, можно записать так:
[0-9a-fA-F]{2}(?:[:-][0-9a-fA-F]{2}){5}

Задача 04. Замена времени

Задача 05. Действительные числа в паскале

Задача 06. Аббревиатуры

Задача 07. Шифровка

Задача 08. То ли акростих, то ли акроним, то ли апроним

Задача 09. Хайку

На всякий случай ещё раз. Каждый их этих шаблонов проверяет лишь то, что идёт непосредственно перед позицией или непосредственно после позиции. Если пару таких шаблонов написать рядом, то проверки будут независимы (то есть будут соответствовать AND в каком-то смысле).

Задача 10. CamelCase -> under_score

Задача 11. Удаление повторов

Задача 12. Близкие слова

Задача 13. Форматирование больших чисел

Задача 14. Разделить текст на предложения

Задача 15. Форматирование номера телефона

Задача 16. Поиск e-mail’ов — 2