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Python RegEx

A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern.

RegEx can be used to check if a string contains the specified search pattern.

RegEx Module

Python has a built-in package called re, which can be used to work with Regular Expressions.

Import the re module:

RegEx in Python

When you have imported the re module, you can start using regular expressions:

Example

Search the string to see if it starts with "The" and ends with "Spain":

import re

txt = "The rain in Spain"
x = re.search("^The.*Spain$", txt)

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RegEx Functions

The re module offers a set of functions that allows us to search a string for a match:

Function	Description
findall	Returns a list containing all matches
search	Returns a Match object if there is a match anywhere in the string
split	Returns a list where the string has been split at each match
sub	Replaces one or many matches with a string

Metacharacters

Metacharacters are characters with a special meaning:

Character	Description	Example	Try it
[]	A set of characters	"[a-m]"	Try it »
\	Signals a special sequence (can also be used to escape special characters)	"\d"	Try it »
.	Any character (except newline character)	"he..o"	Try it »
^	Starts with	"^hello"	Try it »
$	Ends with	"planet$"	Try it »
*	Zero or more occurrences	"he.*o"	Try it »
+	One or more occurrences	"he.+o"	Try it »
?	Zero or one occurrences	"he.?o"	Try it »
{}	Exactly the specified number of occurrences	"he.{2}o"	Try it »
\|	Either or	"falls\|stays"	Try it »
()	Capture and group

Flags

You can add flags to the pattern when using regular expressions.

Flag	Shorthand	Description	Try it
re.ASCII	re.A	Returns only ASCII matches	Try it »
re.DEBUG	Returns debug information	Try it »
re.DOTALL	re.S	Makes the . character match all characters (including newline character)	Try it »
re.IGNORECASE	re.I	Case-insensitive matching	Try it »
re.MULTILINE	re.M	Returns only matches at the beginning of each line	Try it »
re.NOFLAG	Specifies that no flag is set for this pattern
re.UNICODE	re.U	Returns Unicode matches. This is default from Python 3. For Python 2: use this flag to return only Unicode matches	Try it »
re.VERBOSE	re.X	Allows whitespaces and comments inside patterns. Makes the pattern more readable	Try it »

Special Sequences

A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:

Character	Description	Example	Try it
\A	Returns a match if the specified characters are at the beginning of the string	"\AThe"	Try it »
\b	Returns a match where the specified characters are at the beginning or at the end of a word(the "r" in the beginning is making sure that the string is being treated as a "raw string")	r"\bain"r"ain\b"	Try it » Try it »
\B	Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word(the "r" in the beginning is making sure that the string is being treated as a "raw string")	r"\Bain"r"ain\B"	Try it » Try it »
\d	Returns a match where the string contains digits (numbers from 0-9)	"\d"	Try it »
\D	Returns a match where the string DOES NOT contain digits	"\D"	Try it »
\s	Returns a match where the string contains a white space character	"\s"	Try it »
\S	Returns a match where the string DOES NOT contain a white space character	"\S"	Try it »
\w	Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character)	"\w"	Try it »
\W	Returns a match where the string DOES NOT contain any word characters	"\W"	Try it »
\Z	Returns a match if the specified characters are at the end of the string	"Spain\Z"	Try it »

Sets

A set is a set of characters inside a pair of square brackets [] with a special meaning:

Set	Description	Try it
[arn]	Returns a match where one of the specified characters (a,r, or n) is present	Try it »
[a-n]	Returns a match for any lower case character, alphabetically betweena and n	Try it »
[^arn]	Returns a match for any character EXCEPT a,r, and n	Try it »
[0123]	Returns a match where any of the specified digits (0,1, 2, or 3) are present	Try it »
[0-9]	Returns a match for any digit between0 and 9	Try it »
[0-5][0-9]	Returns a match for any two-digit numbers from 00 and 59	Try it »
[a-zA-Z]	Returns a match for any character alphabetically betweena and z, lower case OR upper case	Try it »
[+]	In sets, +, *,., \|,(), $,{}has no special meaning, so [+] means: return a match for any+ character in the string	Try it »

The findall() Function

The findall() function returns a list containing all matches.

Example

Print a list of all matches:

import re

txt = "The rain in Spain"
x = re.findall("ai", txt)
print(x)

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The list contains the matches in the order they are found.

If no matches are found, an empty list is returned:

Example

Return an empty list if no match was found:

import re

txt = "The rain in Spain"
x = re.findall("Portugal", txt)
print(x)

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The search() Function

The search() function searches the string for a match, and returns a Match object if there is a match.

If there is more than one match, only the first occurrence of the match will be returned:

Example

Search for the first white-space character in the string:

import re

txt = "The rain in Spain"
x = re.search("\s", txt)

print("The first white-space character is located in position:", x.start())

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If no matches are found, the value None is returned:

Example

Make a search that returns no match:

import re

txt = "The rain in Spain"
x = re.search("Portugal", txt)
print(x)

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The split() Function

The split() function returns a list where the string has been split at each match:

Example

Split at each white-space character:

import re

txt = "The rain in Spain"
x = re.split("\s", txt)
print(x)

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You can control the number of occurrences by specifying the maxsplitparameter:

Example

Split the string only at the first occurrence:

import re

txt = "The rain in Spain"
x = re.split("\s", txt, 1)
print(x)

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The sub() Function

The sub() function replaces the matches with the text of your choice:

Example

Replace every white-space character with the number 9:

import re

txt = "The rain in Spain"
x = re.sub("\s", "9", txt)
print(x)

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You can control the number of replacements by specifying thecountparameter:

Example

Replace the first 2 occurrences:

import re

txt = "The rain in Spain"
x = re.sub("\s", "9", txt, 2)
print(x)

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Match Object

A Match Object is an object containing information about the search and the result.

Note: If there is no match, the value None will be returned, instead of the Match Object.

Example

Do a search that will return a Match Object:

import re

txt = "The rain in Spain"
x = re.search("ai", txt)
print(x) #this will print an object

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The Match object has properties and methods used to retrieve information about the search, and the result:

.span() returns a tuple containing the start-, and end positions of the match.
.string returns the string passed into the function
.group() returns the part of the string where there was a match