RegEx Python | Python Tutorials

RegEx

A RegEx(Regular Expression) is a sequence of characters that forms a search pattern. It 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

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


import re

#Check if the string starts with "The" and ends with "Spain":

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

if x:
  print("YES! We have a match!")
else:
  print("No match")

Output:


YES! We have a match!

RegEx Functions

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

Character	Description	Example
[]	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	"world$"
*	Zero or more occurrences	"aix*"
+	One or more occurrences	"aix+"
{}	Exactly the specified number of occurrences	"al{2}"
\|	Either or	"falls\|stays"
()	Capture and group

Special Sequences

Character	Description	Example
\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"ain\b"
\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"
\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	"Spain\Z"

Sets

class="w3-codespan">a, r, or n) are present

Set	Description
[arn]	Returns a match where one of the specified characters (`a`, `r`, or `n`) are 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

findall() Function

findall() function returns a list containing all matches

Example
Print a list of all matches:


import re

#Return a list containing every occurrence of "ai":

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

Output:


['ai', 'ai']

The list contains the matches in the order they are found. If no matches are found, an empty list is returned

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:


import re

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

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

Output:


The first white-space character is located in position: 3

If no matches are found, the value None is returned

split() Function

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


import re

#Split the string at every white-space character:

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

Output:


['The', 'rain', 'in', 'Spain']

sub() Function

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


import re

#Replace all white-space characters with the digit "9":

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

Output:


The9rain9in9Spain

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


import re

#The search() function returns a Match object:

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

Output:


<_sre.SRE_Match object; span=(5, 7), match='ai'>

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