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)

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

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)

['Python', 'Python']

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())

16

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)

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

Split with Limit

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

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

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)

My pin is ****

Limit Replacements

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

Call XXXX543210 now

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())

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

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