Python RegEx

In this tutorial, you will learn about regular expressions (RegEx), and use Python's re module to work with RegEx (with the help of examples).

A Regular Expression (RegEx) is a sequence of characters that defines a search pattern. For example,

^a...s$

The above code defines a RegEx pattern. The pattern is: any five letter string starting with a and ending with s.

A pattern defined using RegEx can be used to match against a string.

Expression	String	Matched?
^a...s$	abs	No match
alias	Match
abyss	Match
Alias	No match
An abacus	No match

Python has a module named re to work with RegEx. Here's an example:

import re

pattern = '^a...s$'
test_string = 'abyss'
result = re.match(pattern, test_string)

if result:
  print("Search successful.")
else:
  print("Search unsuccessful.")	

Here, we used re.match() function to search pattern within the test_string. The method returns a match object if the search is successful. If not, it returns None.

There are other several functions defined in the re module to work with RegEx. Before we explore that, let's learn about regular expressions themselves.

If you already know the basics of RegEx, jump to Python RegEx.

Specify Pattern Using RegEx

To specify regular expressions, metacharacters are used. In the above example, ^ and $ are metacharacters.

MetaCharacters

Metacharacters are characters that are interpreted in a special way by a RegEx engine. Here's a list of metacharacters:

[] . ^ $ * + ? {} () \ |

[] - Square brackets

Square brackets specifies a set of characters you wish to match.

Expression	String	Matched?
[abc]	a	1 match
ac	2 matches
Hey Jude	No match
abc de ca	5 matches

Here, [abc] will match if the string you are trying to match contains any of the a, b or c.

You can also specify a range of characters using - inside square brackets.

• [a-e] is the same as [abcde].

• [1-4] is the same as [1234].

• [0-39] is the same as [01239].

You can complement (invert) the character set by using caret ^ symbol at the start of a square-bracket.

• [^abc] means any character except a or b or c.

• [^0-9] means any non-digit character.

. - Period

A period matches any single character (except newline '\n').

Expression	String	Matched?
..	a	No match
ac	1 match
acd	1 match
acde	2 matches (contains 4 characters)

^ - Caret

The caret symbol ^ is used to check if a string starts with a certain character.

Expression	String	Matched?
^a	a	1 match
abc	1 match
bac	No match
^ab	abc	1 match
acb	No match (starts with a but not followed by b)

$ - Dollar

The dollar symbol $ is used to check if a string ends with a certain character.

Expression	String	Matched?
a$	a	1 match
formula	1 match
cab	No match

* - Star

The star symbol * matches zero or more occurrences of the pattern left to it.

Expression	String	Matched?
ma*n	mn	1 match
man	1 match
maaan	1 match
main	No match (a is not followed by n)
woman	1 match

+ - Plus

The plus symbol + matches one or more occurrences of the pattern left to it.

Expression	String	Matched?
ma+n	mn	No match (no a character)
man	1 match
maaan	1 match
main	No match (a is not followed by n)
woman	1 match

? - Question Mark

The question mark symbol ? matches zero or one occurrence of the pattern left to it.

Expression	String	Matched?
ma?n	mn	1 match
man	1 match
maaan	No match (more than one a character)
main	No match (a is not followed by n)
woman	1 match

{} - Braces

Consider this code: {n,m}. This means at least n, and at most m repetitions of the pattern left to it.

Expression	String	Matched?
a{2,3}	abc dat	No match
abc daat	1 match (at daat)
aabc daaat	2 matches (at aabc and daaat)
aabc daaaat	2 matches (at aabc and daaaat)

Let's try one more example. This RegEx [0-9]{2, 4} matches at least 2 digits but not more than 4 digits

Expression	String	Matched?
[0-9]{2,4}	ab123csde	1 match (match at ab123csde)
12 and 345673	3 matches (12, 3456, 73)
1 and 2	No match

| - Alternation

Vertical bar | is used for alternation (or operator).

Expression	String	Matched?
a|b	cde	No match
ade	1 match (match at ade)
acdbea	3 matches (at acdbea)

Here, a|b match any string that contains either a or b

() - Group

Parentheses () is used to group sub-patterns. For example, (a|b|c)xz match any string that matches either a or b or c followed by xz

Expression	String	Matched?
(a|b|c)xz	ab xz	No match
abxz	1 match (match at abxz)
axz cabxz	2 matches (at axzbc cabxz)

\ - Backslash

Backlash \ is used to escape various characters including all metacharacters. For example,

\$a match if a string contains $ followed by a. Here, $ is not interpreted by a RegEx engine in a special way.

If you are unsure if a character has special meaning or not, you can put \ in front of it. This makes sure the character is not treated in a special way.

Special Sequences

Special sequences make commonly used patterns easier to write. Here's a list of special sequences:

\A - Matches if the specified characters are at the start of a string.

Expression	String	Matched?
\Athe	the sun	Match
In the sun	No match

\b - Matches if the specified characters are at the beginning or end of a word.

Expression	String	Matched?
\bfoo	football	Match
a football	Match
afootball	No match
foo\b	the foo	Match
the afoo test	Match
the afootest	No match

\B - Opposite of \b. Matches if the specified characters are not at the beginning or end of a word.

Expression	String	Matched?
\Bfoo	football	No match
a football	No match
afootball	Match
foo\B	the foo	No match
the afoo test	No match
the afootest	Match

\d - Matches any decimal digit. Equivalent to [0-9]

Expression	String	Matched?
\d	12abc3	3 matches (at 12abc3)
Python	No match

\D - Matches any non-decimal digit. Equivalent to [^0-9]

Expression	String	Matched?
\D	1ab34"50	3 matches (at 1ab34"50)
1345	No match

\s - Matches where a string contains any whitespace character. Equivalent to [ \t\n\r\f\v].

Expression	String	Matched?
\s	Python RegEx	1 match
PythonRegEx	No match

\S - Matches where a string contains any non-whitespace character. Equivalent to [^ \t\n\r\f\v].

Expression	String	Matched?
\S	a b	2 matches (at a b)
	No match

\w - Matches any alphanumeric character (digits and alphabets). Equivalent to [a-zA-Z0-9_]. By the way, underscore _ is also considered an alphanumeric character.

Expression	String	Matched?
\w	12&": ;c	3 matches (at 12&": ;c)
%"> !	No match

\W - Matches any non-alphanumeric character. Equivalent to [^a-zA-Z0-9_]

Expression	String	Matched?
\W	1a2%c	1 match (at 1a2%c)
Python	No match

\Z - Matches if the specified characters are at the end of a string.

Expression	String	Matched?
Python\Z	I like Python	1 match
I like Python Programming	No match
Python is fun.	No match

Tip: To build and test regular expressions, you can use RegEx tester tools such as regex101. This tool not only helps you in creating regular expressions, but it also helps you learn it.

Now you understand the basics of RegEx, let's discuss how to use RegEx in your Python code.

Python RegEx

Python has a module named re to work with regular expressions. To use it, we need to import the module.

import re

The module defines several functions and constants to work with RegEx.

re.findall()

The re.findall() method returns a list of strings containing all matches.

Example 1: re.findall()

# Program to extract numbers from a string

import re

string = 'hello 12 hi 89. Howdy 34'
pattern = '\d+'

result = re.findall(pattern, string) 
print(result)

# Output: ['12', '89', '34']

If the pattern is not found, re.findall() returns an empty list.

re.split()

The re.split method splits the string where there is a match and returns a list of strings where the splits have occurred.

Example 2: re.split()

import re

string = 'Twelve:12 Eighty nine:89.'
pattern = '\d+'

result = re.split(pattern, string) 
print(result)

# Output: ['Twelve:', ' Eighty nine:', '.']

If the pattern is not found, re.split() returns a list containing the original string.

You can pass maxsplit argument to the re.split() method. It's the maximum number of splits that will occur.

import re

string = 'Twelve:12 Eighty nine:89 Nine:9.'
pattern = '\d+'

# maxsplit = 1
# split only at the first occurrence
result = re.split(pattern, string, 1) 
print(result)

# Output: ['Twelve:', ' Eighty nine:89 Nine:9.']

By the way, the default value of maxsplit is 0; meaning all possible splits.

re.sub()

The syntax of re.sub() is:

re.sub(pattern, replace, string)

The method returns a string where matched occurrences are replaced with the content of replace variable.

Example 3: re.sub()

# Program to remove all whitespaces
import re

# multiline string
string = 'abc 12\
de 23 \n f45 6'

# matches all whitespace characters
pattern = '\s+'

# empty string
replace = ''

new_string = re.sub(pattern, replace, string) 
print(new_string)

# Output: abc12de23f456

If the pattern is not found, re.sub() returns the original string.

You can pass count as a fourth parameter to the re.sub() method. If omited, it results to 0. This will replace all occurrences.

import re

# multiline string
string = 'abc 12\
de 23 \n f45 6'

# matches all whitespace characters
pattern = '\s+'
replace = ''

new_string = re.sub(r'\s+', replace, string, 1) 
print(new_string)

# Output:
# abc12de 23
# f45 6

re.subn()

The re.subn() is similar to re.sub() expect it returns a tuple of 2 items containing the new string and the number of substitutions made.

Example 4: re.subn()

# Program to remove all whitespaces
import re

# multiline string
string = 'abc 12\
de 23 \n f45 6'

# matches all whitespace characters
pattern = '\s+'

# empty string
replace = ''

new_string = re.subn(pattern, replace, string) 
print(new_string)

# Output: ('abc12de23f456', 4)

re.search()

The re.search() method takes two arguments: a pattern and a string. The method looks for the first location where the RegEx pattern produces a match with the string.

If the search is successful, re.search() returns a match object; if not, it returns None.

match = re.search(pattern, str)

Example 5: re.search()

import re

string = "Python is fun"

# check if 'Python' is at the beginning
match = re.search('\APython', string)

if match:
  print("pattern found inside the string")
else:
  print("pattern not found")  

# Output: pattern found inside the string

Here, match contains a match object.

Match object

You can get methods and attributes of a match object using dir() function.

Some of the commonly used methods and attributes of match objects are:

match.group()

The group() method returns the part of the string where there is a match.

Example 6: Match object

import re

string = '39801 356, 2102 1111'

# Three digit number followed by space followed by two digit number
pattern = '(\d{3}) (\d{2})'

# match variable contains a Match object.
match = re.search(pattern, string) 

if match:
  print(match.group())
else:
  print("pattern not found")

# Output: 801 35

Here, match variable contains a match object.

Our pattern (\d{3}) (\d{2}) has two subgroups (\d{3}) and (\d{2}). You can get the part of the string of these parenthesized subgroups. Here's how:

>>> match.group(1)
'801'

>>> match.group(2)
'35'
>>> match.group(1, 2)
('801', '35')

>>> match.groups()
('801', '35')

match.start(), match.end() and match.span()

The start() function returns the index of the start of the matched substring. Similarly, end() returns the end index of the matched substring.

>>> match.start()
2
>>> match.end()
8

The span() function returns a tuple containing start and end index of the matched part.

>>> match.span()
(2, 8)

match.re and match.string

The re attribute of a matched object returns a regular expression object. Similarly, string attribute returns the passed string.

>>> match.re
re.compile('(\\d{3}) (\\d{2})')

>>> match.string
'39801 356, 2102 1111'

We have covered all commonly used methods defined in the re module. If you want to learn more, visit Python 3 re module.

Using r prefix before RegEx

When r or R prefix is used before a regular expression, it means raw string. For example, '\n' is a new line whereas r'\n' means two characters: a backslash \ followed by n.

Backlash \ is used to escape various characters including all metacharacters. However, using r prefix makes \ treat as a normal character.

Example 7: Raw string using r prefix

import re

string = '\n and \r are escape sequences.'

result = re.findall(r'[\n\r]', string) 
print(result)

# Output: ['\n', '\r']

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