Regular Expressions

A Regular Expression, also called RegEx, is a sequence of characters that forms a search pattern for text.

The basics about Regular Expressions

Python supports regular expressions with a built-in module called re. In the example below we will use the re module to search for a pattern in a string and print the result.

Example:

#!/usr/bin/env python3

import re

def main():
    txt = "Hello great world"
    x = re.search("^Hello.*world$", txt)
    if x:
        print("YES! We have a match!")
    else:
        print("No match")

if __name__ == "__main__":
    main()

Output:

YES! We have a match!

Metacharacters

In the previous example we used the ^ and $ metacharacters to search for a pattern at the beginning and end of a string. Beside these two metacharacters there are other metacharacters that we will use in the next examples and are listed in the table below.

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	"error|failed"
()	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
\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
[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

Example:

#!/usr/bin/env python3

import re

def main():
    txt = "Hello great world"
    x = re.findall("[arn]", txt)
    print(x)

if __name__ == "__main__":
    main()

Regular Expressions functions

findall()

Returns a list containing all matches

Example:

#!/usr/bin/env python3

import re

def main():
    txt = "Hello great world"
    x = re.findall("ea", txt)
    print(x)

if __name__ == "__main__":
    main()

search()

Returns a Match object if there is a match anywhere in the string

Example:

#!/usr/bin/env python3

import re

def main():
    txt = "Hello great world"
    x = re.search("\s", txt)
    print("The first white-space character is located in position:", x.start())

if __name__ == "__main__":
    main()

split()

Returns a list where the string has been split at each match

Example:

#!/usr/bin/env python3

import re

def main():
    txt = "Hello great world"
    x = re.split("\s", txt)
    print(x)

if __name__ == "__main__":
    main()

sub()

Replaces one or many matches with a string

Example:

#!/usr/bin/env python3

import re

def main():
    txt = "Hello great world"
    x = re.sub("\s", "9", txt)
    print(x)

if __name__ == "__main__":
    main()

Example:

#!/usr/bin/env python3

import re

def main():
    txt = "Hello great world"
    x = re.sub("\s", "9", txt, 2)
    print(x)

if __name__ == "__main__":
    main()

matching

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:

#!/usr/bin/env python3

import re

def main():
    txt = "Hello great world"
    x = re.search("ea", txt)
    print(x)

if __name__ == "__main__":
    main()

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

span()

Example:

#!/usr/bin/env python3

import re

def main():
    txt = "Hello great world"
    x = re.search(r"\bS\w+", txt)
    print(x.span())

if __name__ == "__main__":
    main()

string()

Example:

#!/usr/bin/env python3

import re

def main():
    txt = "Hello great world"
    x = re.search(r"\bS\w+", txt)
    print(x.string())

if __name__ == "__main__":
    main()

group()

Example:

#!/usr/bin/env python3

import re

def main():
    txt = "Hello great world"
    x = re.search(r"\bS\w+", txt)
    print(x.group())

if __name__ == "__main__":
    main()