Regular Expressions

Regular expressions (often called regex) let you search, extract, and match text patterns inside Python strings. They give you a quick way to check whether a string matches a specific rule or format.

Related Course: Python Strings and Data Structures

In Python, you use the standard re module. You can find a complete syntax reference at the bottom of this page.

The Match function

The match function is defined as:

re.match(pattern, string)

The parameters are:

If you want to match a string to a numberic sequence of exactly five, you can use this code:

#!/usr/bin/python
import re
input = raw_input("Enter an input string:")
m = re.match('\d{5}\Z',input)
if m:
    print("True")
else:
    print("False")

Example outputs:

Parameters	Description
pattern	a regular expression
string	the input string

Email validation regex

We can use the same function to validate email address. The grammar rules are seen in re.compile and in the grammar table.

#!/usr/bin/python
import re
input = raw_input("Enter an input string:")
m = re.match('[^@]+@[^@]+\.[^@]+',input)
if m:
    print("True")
else:
    print("False")

The Search Function

The search function is defined as:

re.search(pattern, string)

The parameters are:

String	Match
12345	True
12358	True
55555	True
123	False
123K5	False
5555555	False

To search if an e-mail address is in a string:

#!/usr/bin/python
import re
input = "Contact me by [email protected] or at the office."
m = re.search('[^@]+@[^@]+\.[^@]+',input)
if m:
    print("String found.")
else:
    print("Nothing found.")

Regular Expression Examples

A few examples of regular expressions:

Parameter	Description
pattern	a regular expression, defines the string to be searched
string	the search space

Regular Expression Grammar

Overview of the regex grammar:

Example	Regex
IP address	(([2][5][0-5]\.)|([2][0-4][0-9]\.)|([0-1]?[0-9]?[0-9]\.)){3}(([2][5][0-5])|([2][0-4][0-9])|([0-1]?[0-9]?[0-9]))
Email	[^@]+@[^@]+\.[^@]+
Date MM/DD/YY	(\d+/\d+/\d+)
Integer (positive)	(?<![-.])\b[0-9]+\b(?!\.[0-9])
Integer	[+-]?(?<!\.)\b[0-9]+\b(?!\.[0-9])
Float	(?<=>)\d+.\d+|\d+
Hexadecimal	\s–([0-9a-fA-F]+)(?:–)?\s

Regex	Description
\d	Matches any decimal digit; this is equivalent to the class [0-9]
\D	Matches any non-digit character; this is equivalent to the class [^0-9].
\s	Matches any whitespace character; this is equivalent to the class [ \n\r\f\v].
\S	Matches any non-whitespace character; this is equivalent to the class [^ \n\r\f\v].
\w	Matches any alphanumeric character; this is equivalent to the class [a-zA-Z0-9_].
\W	Matches any non-alphanumeric character; this is equivalent to the class [^a-zA-Z0-9_].
\Z	Matches only at end of string
[..]	Match single character in brackets
[^..]	Match any single character not in brackets
.	Match any character except newline
$	Match the end of the string
*	Match 0 or more repetitions
+	1 or more repetitions
{m}	Exactly m copies of the previous RE should be matched.
|	Match A or B. A|B
?	0 or 1 repetitions of the preceding RE
[a-z]	Any lowercase character
[A-Z]	Any uppercase character
[a-zA-Z]	Any character
[0-9]	Any digit