pyregex example

Above visualization created using debuggex for the pattern r'\bpar(en|ro)?t\b'

From docs.python: re:

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

This blog post gives an overview and examples of regular expression syntax as implemented by the re built-in module (Python 3.11+). Assume ASCII character set unless otherwise specified. This post is an excerpt from my Understanding Python re(gex)? book.

Elements that define a regular expression🔗

\Arestricts the match to the start of string
\Zrestricts the match to the end of string
^restricts the match to the start of line
$restricts the match to the end of line
\nnewline character is used as line separator
re.MULTILINE or re.Mflag to treat input as multiline string
\brestricts the match to the start/end of words
word characters: alphabets, digits, underscore
\Bmatches wherever \b doesn't match

^, $ and \ are metacharacters in the above table, as these characters have special meaning. Prefix a \ character to remove the special meaning and match such characters literally. For example, \^ will match a ^ character instead of acting as an anchor.

|multiple RE combined as conditional OR
each alternative can have independent anchors
(pat)group pattern(s), also a capturing group
a(b|c)d is same as abd|acd
(?:pat)non-capturing group
(?P<name>pat)named capture group
.Match any character except the newline character \n
[]Character class, matches one character among many

Greedy QuantifiersDescription
*Match zero or more times
+Match one or more times
?Match zero or one times
{m,n}Match m to n times (inclusive)
{m,}Match at least m times
{,n}Match up to n times (including 0 times)
{n}Match exactly n times
pat1.*pat2any number of characters between pat1 and pat2
pat1.*pat2|pat2.*pat1match both pat1 and pat2 in any order

Greedy here means that the above quantifiers will match as much as possible that'll also honor the overall RE. Appending a ? to greedy quantifiers makes them non-greedy, i.e. match as minimally as possible. Appending a + to greedy quantifiers makes them possessive, which prevents backtracking. You can also use (?>pat) atomic grouping to safeguard from backtracking. Quantifiers can be applied to literal characters, groups, backreferences and character classes.

Character classDescription
[aeiou]Match any vowel
[^aeiou]^ inverts selection, so this matches any consonant
[a-f]- defines a range, so this matches any of abcdef characters
\dMatch a digit, same as [0-9]
\DMatch non-digit, same as [^0-9] or [^\d]
\wMatch word character, same as [a-zA-Z0-9_]
\WMatch non-word character, same as [^a-zA-Z0-9_] or [^\w]
\sMatch whitespace character, same as [\ \t\n\r\f\v]
\SMatch non-whitespace character, same as [^\ \t\n\r\f\v] or [^\s]

lookaroundscustom assertions, zero-width like anchors
(?!pat)negative lookahead assertion
(?<!pat)negative lookbehind assertion
(?=pat)positive lookahead assertion
(?<=pat)positive lookbehind assertion
(?!pat1)(?=pat2)multiple assertions can be specified in any order
as they mark a matching location without consuming characters
((?!pat).)*Negate a grouping, similar to negated character class

re.IGNORECASE or re.Iflag to ignore case
re.DOTALL or re.Sallow . metacharacter to match newline character
flags=re.S|re.Imultiple flags can be combined using | operator
re.MULTILINE or re.Mallow ^ and $ anchors to match line wise
re.VERBOSE or re.Xallows to use literal whitespaces for aligning purposes
and to add comments after the # character
escape spaces and # if needed as part of actual RE
re.ASCII or re.Amatch only ASCII characters for \b, \w, \d, \s
and their opposites, applicable only for Unicode patterns
re.LOCALE or re.Luse locale settings for byte patterns and 8-bit locales
(?#comment)another way to add comments, not a flag
(?flags:pat)inline flags only for this pat, overrides flags argument
flags is i for re.I, s for re.S, etc, except L for re.L
(?-flags:pat)negate flags only for this pat
(?flags-flags:pat)apply and negate particular flags only for this pat
(?flags)apply flags for whole RE, can be used only at start of RE
anchors if any, should be specified after (?flags)

Matched portionDescription
re.Match objectdetails like matched portions, location, etc
m[0] or matched portion of re.Match object m
m[n] or portion of nth capture group
m.groups()tuple of all the capture groups' matched portions
m.span()start and end+1 index of entire matched portion
pass a number to get span of that particular capture group
can also use m.start() and m.end()
\Nbackreference, gives matched portion of Nth capture group
applies to both search and replacement sections
possible values: \1, \2 up to \99 provided no more digits
\g<N>backreference, gives matched portion of Nth capture group
possible values: \g<0>, \g<1>, etc (not limited to 99)
\g<0> refers to entire matched portion
(?P<name>pat)named capture group
refer as 'name' in re.Match object
refer as (?P=name) in search section
refer as \g<name> in replacement section
groupdictmethod applied on a re.Match object
gives named capture group portions as a dict

info \0 and \100 onwards are considered as octal values, hence cannot be used as backreferences.

re module functions🔗

re.searchCheck if given pattern is present anywhere in input string
Output is a re.Match object, usable in conditional expressions
r-strings preferred to define RE
Use byte pattern for byte input
Python also maintains a small cache of recent RE
re.fullmatchensures pattern matches the entire input string
re.compileCompile a pattern for reuse, outputs re.Pattern object
re.subsearch and replace
re.sub(r'pat', f, s)function f with re.Match object as argument
re.escapeautomatically escape all metacharacters
re.splitsplit a string based on RE
text matched by the groups will be part of the output
portion matched by pattern outside group won't be in output
re.findallreturns all the matches as a list
if 1 capture group is used, only its matches are returned
1+, each element will be tuple of capture groups
portion matched by pattern outside group won't be in output
re.finditeriterator with re.Match object for each match
re.subngives tuple of modified string and number of substitutions

The function definitions are given below:, string, flags=0)
re.fullmatch(pattern, string, flags=0)
re.compile(pattern, flags=0)
re.sub(pattern, repl, string, count=0, flags=0)
re.split(pattern, string, maxsplit=0, flags=0)
re.findall(pattern, string, flags=0)
re.finditer(pattern, string, flags=0)
re.subn(pattern, repl, string, count=0, flags=0)

Regular expression examples🔗

As a good practice, always use raw strings to construct RE, unless other formats are required. This will avoid conflict between special meaning of the backslash character in RE and string literals.

info I wrote an interactive TUI app to help you experiment with the examples presented below. See PyRegexPlayground repo for installation instructions and usage guide. See PyRegexExercises repo for a TUI app with 100+ Python regex exercises.

  • examples for
>>> sentence = 'This is a sample string'

# need to load the re module before use
>>> import re
# check if 'sentence' contains the pattern described by RE argument
>>> bool('is', sentence))

# ignore case while searching for a match
>>> bool('this', sentence, flags=re.I))

# example when pattern isn't found in the input string
>>> bool('xyz', sentence))

# output can be directly used in conditional expressions
>>> if'ring', sentence):
...     print('mission success')
mission success

# use raw byte strings for patterns if input is of byte data type
>>> bool('is', b'This is a sample string'))
  • string and line anchors
# match the start of the input string
>>> bool('\Ahi', 'hi hello\ntop spot'))

# match the start of a line
>>> bool('^top', 'hi hello\ntop spot', flags=re.M))

# match the end of strings
>>> words = ['surrender', 'up', 'newer', 'do', 'era', 'eel', 'pest']
>>> [w for w in words if'er\Z', w)]
['surrender', 'newer']

# check if there's a whole line 'par'
>>> bool('^par$', 'spare\npar\ndare', flags=re.M))
  • examples for re.findall()
# match 'par' with optional 's' at start and optional 'e' at end
>>> re.findall(r'\bs?pare?\b', 'par spar apparent spare part pare')
['par', 'spar', 'spare', 'pare']

# numbers >= 100 with optional leading zeros
# use r'\b0*[1-9]\d{2,}\b' if possessive quantifiers isn't supported
>>> re.findall(r'\b0*+\d{3,}\b', '0501 035 154 12 26 98234')
['0501', '154', '98234']

# if multiple capturing groups are used, each element of output
# will be a tuple of strings of all the capture groups
>>> re.findall(r'([^/]+)/([^/,]+),?', '2020/04,1986/Mar')
[('2020', '04'), ('1986', 'Mar')]

# normal capture group will hinder ability to get whole match
# non-capturing group to the rescue
>>> re.findall(r'\b\w*(?:st|in)\b', 'cost akin more east run')
['cost', 'akin', 'east']

# useful for debugging purposes as well
>>> re.findall(r':.*?:', 'green:3.14:teal::brown:oh!:blue')
[':3.14:', '::', ':oh!:']
  • examples for re.split()
# split based on one or more digit characters
>>> re.split(r'\d+', 'Sample123string42with777numbers')
['Sample', 'string', 'with', 'numbers']

# split based on digit or whitespace characters
>>> re.split(r'[\d\s]+', '**1\f2\n3star\t7 77\r**')
['**', 'star', '**']

# to include the matching delimiter strings as well in the output
>>> re.split(r'(\d+)', 'Sample123string42with777numbers')
['Sample', '123', 'string', '42', 'with', '777', 'numbers']

# multiple capture groups example
# note that the portion matched by b+ isn't present in the output
>>> re.split(r'(a+)b+(c+)', '3.14aabccc42')
['3.14', 'aa', 'ccc', '42']

# use non-capturing group if capturing is not needed
>>> re.split(r'hand(?:y|ful)', '123handed42handy777handful500')
['123handed42', '777', '500']
  • backreferencing within the search pattern
# whole words that have at least one consecutive repeated character
>>> words = ['effort', 'flee', 'facade', 'oddball', 'rat', 'tool']

>>> [w for w in words if'\b\w*(\w)\1\w*\b', w)]
['effort', 'flee', 'oddball', 'tool']
  • working with matched portions
# re.Match object
>>>'so+n', 'too soon a song snatch')
<re.Match object; span=(4, 8), match='soon'>

# retrieving entire matched portion, note the use of [0]
>>> motivation = 'Doing is often better than thinking of doing.'
>>>'of.*ink', motivation)[0]
'often better than think'

# capture group example
>>> purchase = 'coffee:100g tea:250g sugar:75g chocolate:50g'
>>> m =':(.*?)g.*?:(.*?)g.*?chocolate:(.*?)g', purchase)
# to get the matched portion of the second capture group
>>> m[2]

# to get a tuple of all the capture groups
>>> m.groups()
('100', '250', '50')
  • examples for re.finditer()
# numbers < 350
>>> m_iter = re.finditer(r'[0-9]+', '45 349 651 593 4 204 350')
>>> [m[0] for m in m_iter if int(m[0]) < 350]
['45', '349', '4', '204']

# start and end+1 index of each matching portion
>>> m_iter = re.finditer(r'so+n', 'song too soon snatch')
>>> for m in m_iter:
...     print(m.span())
(0, 3)
(9, 13)
  • examples for re.sub()
# add something to the start of every line
>>> ip_lines = "catapults\nconcatenate\ncat"
>>> print(re.sub(r'^', '* ', ip_lines, flags=re.M))
* catapults
* concatenate
* cat

# replace 'par' only at the start of a word
>>> re.sub(r'\bpar', 'X', 'par spar apparent spare part')
'X spar apparent spare Xt'

# same as: r'part|parrot|parent'
>>> re.sub(r'par(en|ro)?t', 'X', 'par part parrot parent')
'par X X X'

# remove first two columns where : is delimiter
>>> re.sub(r'\A([^:]+:){2}', '', 'apple:123:banana:cherry')
  • backreferencing in the replacement section
# remove any number of consecutive duplicate words separated by space
# use \W+ instead of space to cover cases like 'a;a<-;a'
>>> re.sub(r'\b(\w+)( \1)+\b', r'\1', 'aa a a a 42 f_1 f_1 f_13.14')
'aa a 42 f_1 f_13.14'

# add something around the matched strings
>>> re.sub(r'\d+', r'(\g<0>0)', '52 apples and 31 mangoes')
'(520) apples and (310) mangoes'

# swap words that are separated by a comma
>>> re.sub(r'(\w+),(\w+)', r'\2,\1', 'good,bad 42,24')
'bad,good 24,42'

# example with both capturing and non-capturing groups
>>> re.sub(r'(\d+)(?:abc)+(\d+)', r'\2:\1', '1000abcabc42 12abcd21')
'42:1000 12abcd21'
  • using functions in the replacement section of re.sub()
>>> from math import factorial
>>> numbers = '1 2 3 4 5'
>>> def fact_num(n):
...     return str(factorial(int(n[0])))
>>> re.sub(r'\d+', fact_num, numbers)
'1 2 6 24 120'

# using lambda
>>> re.sub(r'\d+', lambda m: str(factorial(int(m[0]))), numbers)
'1 2 6 24 120'
  • examples for lookarounds
# change 'cat' only if it is not followed by a digit character
# note that the end of string satisfies the given assertion
# 'catcat' has two matches as the assertion doesn't consume characters
>>> re.sub(r'cat(?!\d)', 'dog', 'hey cats! cat42 cat_5 catcat')
'hey dogs! cat42 dog_5 dogdog'

# change whole word only if it is not preceded by : or -
>>> re.sub(r'(?<![:-])\b\w+', 'X', ':cart <apple -rest ;tea')
':cart <X -rest ;X'

# extract digits only if it is preceded by - and followed by ; or :
>>> re.findall(r'(?<=-)\d+(?=[:;])', '42 apple-5, fig3; x-83, y-20: f12')

# words containing 'b' and 'e' and 't' in any order
>>> words = ['sequoia', 'questionable', 'exhibit', 'equation']
>>> [w for w in words if'(?=.*b)(?=.*e).*t', w)]
['questionable', 'exhibit']

# match if 'do' is not there between 'at' and 'par'
>>> bool('at((?!do).)*par', 'fox,cat,dog,parrot'))
# match if 'go' is not there between 'at' and 'par'
>>> bool('at((?!go).)*par', 'fox,cat,dog,parrot'))
  • examples for re.compile()

Regular expressions can be compiled using the re.compile() function, which gives back a re.Pattern object. The top level re module functions are all available as methods for this object. Compiling a regular expression helps if the RE has to be used in multiple places or called upon multiple times inside a loop (speed benefit). By default, Python maintains a small list of recently used RE, so the speed benefit doesn't apply for trivial use cases.

>>> pet = re.compile(r'dog')
>>> type(pet)
<class 're.Pattern'>
>>> bool('They bought a dog'))
>>> bool('A cat crossed their path'))

>>> pat = re.compile(r'\([^)]*\)')
>>> pat.sub('', 'a+b(addition) - foo() + c%d(#modulo)')
'a+b - foo + c%d'
>>> pat.sub('', 'Hi there(greeting). Nice day(a(b)')
'Hi there. Nice day'

Understanding Python re(gex)? book🔗

Visit my GitHub repo Understanding Python re(gex)? for details about the book I wrote on Python regular expressions. The book uses plenty of examples to explain the concepts from the basics and introduces more advanced concepts step-by-step. The book also covers the third-party regex module. The cheatsheet and examples presented in this post are based on the contents of this book.

Understanding Python re(gex)? cover image