pyregex example

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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 3.8+). Assume ASCII character set unless otherwise specified. This post is an excerpt from my Python re(gex)? book.

Elements that define a regular expression🔗

AnchorsDescription
\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.

FeatureDescription
|multiple RE combined as conditional OR
each alternative can have independent anchors
(RE)group pattern(s), also a capturing group
a(b|c)d is same as abd|acd
(?:RE)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. 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]
LookaroundsDescription
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
FlagsDescription
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 m.group(0)entire matched portion of re.Match object m
m[n] or m.group(n)matched 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

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

re module functions🔗

FunctionDescription
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:

re.search(pattern, string, flags=0)
re.fullmatch(pattern, string, flags=0)
re.compile(pattern, flags=0)
re.sub(pattern, repl, string, count=0, flags=0)
re.escape(pattern)
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 clash of special meaning of backslash character between RE and normal quoted strings.

  • examples for re.search
>>> 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(re.search(r'is', sentence))
True
# ignore case while searching for a match
>>> bool(re.search(r'this', sentence, flags=re.I))
True
>>> bool(re.search(r'xyz', sentence))
False

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

# use raw byte strings if input is of byte data type
>>> bool(re.search(rb'is', b'This is a sample string'))
True
  • difference between string and line anchors
# string anchors
>>> bool(re.search(r'\Ahi', 'hi hello\ntop spot'))
True
>>> words = ['surrender', 'up', 'newer', 'do', 'ear', 'eel', 'pest']
>>> [w for w in words if re.search(r'er\Z', w)]
['surrender', 'newer']

# line anchors
>>> bool(re.search(r'^par$', 'spare\npar\ndare', flags=re.M))
True
  • examples for re.findall
# whole word 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
>>> re.findall(r'\b0*[1-9]\d{2,}\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't.*?a', 'that is quite a fabricated tale')
['tha', 't is quite a', 'ted ta']
  • 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']

# use non-capturing group if capturing is not needed
>>> re.split(r'hand(?:y|ful)', '123handed42handy777handful500')
['123handed42', '777', '500']
  • backreferencing within 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 re.search(r'\b\w*(\w)\1\w*\b', w)]
['effort', 'flee', 'oddball', 'tool']
  • working with matched portions
>>> re.search(r'b.*d', 'abc ac adc abbbc')
<re.Match object; span=(1, 9), match='bc ac ad'>
# retrieving entire matched portion, note the use of [0]
>>> re.search(r'b.*d', 'abc ac adc abbbc')[0]
'bc ac ad'

# capture group example
>>> m = re.search(r'a(.*)d(.*a)', 'abc ac adc abbbc')
# to get matched portion of second capture group
>>> m[2]
'c a'
# to get a tuple of all the capture groups
>>> m.groups()
('bc ac a', 'c a')
  • examples for re.finditer
# numbers < 350
>>> m_iter = re.finditer(r'[0-9]+', '45 349 651 593 4 204')
>>> [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'ab+c', 'abc ac adc abbbc')
>>> for m in m_iter:
...     print(m.span())
... 
(0, 3)
(11, 16)
  • examples for re.sub
>>> ip_lines = "catapults\nconcatenate\ncat"
>>> print(re.sub(r'^', r'* ', ip_lines, flags=re.M))
* catapults
* concatenate
* cat

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

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

# remove first two columns where : is delimiter
>>> re.sub(r'\A([^:]+:){2}', r'', 'foo:123:bar:baz', count=1)
'bar:baz'
  • backreferencing in replacement section
# remove consecutive duplicate words separated by space
>>> 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'
  • using functions in 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 'foo' only if it is not followed by a digit character
# note that end of string satisfies the given assertion
# foofoo has 2 matches as the assertion doesn't consume characters
>>> re.sub(r'foo(?!\d)', r'baz', 'hey food! foo42 foot5 foofoo')
'hey bazd! foo42 bazt5 bazbaz'

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

# match digits only if it is preceded by - and followed by ; or :
>>> re.findall(r'(?<=-)\d+(?=[:;])', 'fo-5, ba3; x-83, y-20: f12')
['20']

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

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

Regular expressions can be compiled using 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(pet.search('They bought a dog'))
True
>>> bool(pet.search('A cat crossed their path'))
False

>>> 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'

Python re(gex)? book🔗

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

Use this leanpub link for a discounted price.

pyregex cover image