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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:

import re

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)

RegEx Functions

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

Function Description
Returns a list containing all matches
Returns a if there is a match anywhere in the string
Returns a list where the string has been split at each match
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]"
Signals a special sequence (can also be used to escape special characters) "d"
. Any character (except newline character) "he..o"
^ Starts with "^hello"
$ Ends with "planet$"
* Zero or more occurrences "he.*o"
+ One or more occurrences "he.+o"
? Zero or one occurrences "he.?o"
{} Exactly the specified number of occurrences "he.{2}o"
| Either or "falls|stays"
() Capture and group    

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"
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"ainb"
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"ainB"
d Returns a match where the string contains digits (numbers from 0-9) "d"
D Returns a match where the string DOES NOT contain digits "D"
s Returns a match where the string contains a white space character "s"
S Returns a match where the string DOES NOT contain a white space character "S"
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"
W Returns a match where the string DOES NOT contain any word characters "W"
Z Returns a match if the specified characters are at the end of the string "SpainZ"

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

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)

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)

The search() Function

The search() function searches the string for a match, and returns a 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())

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)

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)

You can control the number of occurrences by specifying the maxsplit parameter:

Example

Split the string only at the first occurrence:

import re

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

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)

You can control the number of replacements by specifying the count parameter:

Example

Replace the first 2 occurrences:

import re

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

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

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

Example

Print the position (start- and end-position) of the first match occurrence.

The regular expression looks for any words that starts with an upper case "S":

import re

txt = "The rain in Spain"
x = re.search(r"bSw+", txt)
print(x.span())

Example

Print the string passed into the function:

import re

txt = "The rain in Spain"
x = re.search(r"bSw+", txt)
print(x.string)

Example

Print the part of the string where there was a match.

The regular expression looks for any words that starts with an upper case "S":

import re

txt = "The rain in Spain"
x = re.search(r"bSw+", txt)
print(x.group())

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

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