Regular Expressions (RegEx) in Python

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Regular Expressions (RegEx) in Python

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

It is mainly used for matching, searching, and manipulating text.

With RegEx, you can:

    • Validate input

    • Search text patterns

    • Extract specific data

    • Replace parts of strings

RegEx Module in Python

Python provides a built-in module called re to work with regular expressions.

Importing the re Module

import re

Once imported, you can use RegEx functions to search and manipulate strings.

Using RegEx in Python

Example: Check Pattern at Start and End of a String

The following example checks whether a sentence starts with "Hello" and ends with "World".

import re text = "Hello Python World" pattern = "^Hello.*World$" result = re.search(pattern, text) print(result)

Output:

<re.Match object; span=(0, 18), match='Hello Python World'>

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If a match is found, a Match object is returned; otherwise, None is returned.

RegEx Functions

The re module provides several useful functions:

Function	Description
findall()	Returns all matching patterns as a list
search()	Returns the first match as a Match object
split()	Splits a string based on a pattern
sub()	Replaces matched patterns with new text

Metacharacters

Metacharacters have special meanings in RegEx patterns.

Symbol	Meaning	Example
[]	Set of characters	[a-z]
\	Special sequence	\d
.	Any character	c.t
^	Starts with	^Hi
$	Ends with	end$
*	Zero or more	go*
+	One or more	go+
?	Zero or one	go?
{}	Exact count	\d{3}
|	Either or	a|b
()	Grouping	(abc)

Flags in Regular Expressions

Flags modify how RegEx patterns behave.

Flag	Short	Description
re.IGNORECASE	re.I	Case-insensitive matching
re.MULTILINE	re.M	Match beginning of each line
re.DOTALL	re.S	Dot matches newline
re.ASCII	re.A	ASCII-only matching
re.VERBOSE	re.X	Readable RegEx patterns

Special Sequences

Special sequences start with a backslash \ and have specific meanings.

Sequence	Description	Example
\A	Start of string	\AHello
\b	Word boundary	r"\bcat"
\B	Not word boundary	r"\Bcat"
\d	Digits (0–9)	\d+
\D	Non-digits	\D
\s	Whitespace	\s
\S	Non-whitespace	\S
\w	Word characters	\w+
\W	Non-word characters	\W
\Z	End of string	end\Z

Sets in RegEx

Sets are defined using square brackets [].

Set	Description
[xyz]	Matches x, y, or z
[a-z]	Lowercase letters
[^abc]	Anything except a, b, c
[0-9]	Digits
[A-Za-z]	Upper and lowercase letters
[+]	Matches literal +

The findall() Function

Returns all matches as a list.

Example: Find All Occurrences

import re sentence = "Python is powerful and Python is easy" matches = re.findall("Python", sentence) print(matches)

Output:

['Python', 'Python']

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

The search() Function

Returns the first match found in the string.

Example: Search for First Digit

import re data = "Order number is 4598" match = re.search("\d+", data) print(match.start())

Output:

16

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If no match exists, None is returned.

The split() Function

Splits a string at each match.

Example: Split by Comma

import re text = "apple,banana,orange" result = re.split(",", text) print(result)

Output:

['apple', 'banana', 'orange']

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Split with Limit

import re text = "2024-05-20" result = re.split("-", text, 2) print(result)

Output:

['2024', '05', '20']

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The sub() Function

Replaces matching patterns with new text.

Example: Replace Digits

import re message = "My pin is 1234" result = re.sub("\d", "*", message) print(result)

Output:

My pin is ****

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Limit Replacements

import re message = "Call 9876543210 now" result = re.sub("\d", "X", message, 4) print(result)

Output:

Call XXXX543210 now

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Match Object & Methods

A Match object contains details about the match.

Example: Get Match Information

import re text = "Sun rises in the East" match = re.search(r"\bE\w+", text) # The Match Object print(match) # span() – Start and End Position print(match.span()) # string – Original String print(match.string) # group() – Matched Text print(match.group())

Output:

<re.Match object; span=(17, 21), match='East'>
(17, 21)
Sun rises in the East
East

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