Re Module

Last Updated: 22th August 2025

The re module is used for working with regular expressions in Python. It allows you to search, match, and manipulate text using patterns.

Patterns are:

Symbol	Description	Example	Matches
`^`	Matches the start of the string or line	`^Hello`	`Hello there` (start match)
`$`	Matches the end of the string or line	`end$`	`The end` (end match)
`{m}`	Matches exactly `m` repetitions of the preceding element	`a{3}`	`aaa`
`{m,}`	Matches at least `m` repetitions of the preceding element	`a{2,}`	`aa`, `aaa`
`{(m, n)}`	Matches between `m` and `n` repetitions	`a{(2, 4)}`	`aa`, `aaa`, `aaaa`
`( ... )`	Groups patterns or captures matched substring	`(ab)+`	`ab`, `abab`, `ababab`
`\|`	OR operator: matches either pattern on left or right	`cat\|dog`	`cat` or `dog`
`[abc]`	Character class: matches any one character listed	`[aeiou]`	any vowel
`[^abc]`	Negated character class: matches any character not listed	`[^0-9]`	any non-digit character
`\`	Escape character: matches a literal backslash or special char	`\.`	a literal dot `.`
`.`	Matches any single character except newline	`a.b`	`aab`, `acb`, `a9b`
`*`	Matches zero or more repetitions of the previous character	`ca*t`	`ct`, `cat`, `caaaat`
`+`	Matches one or more repetitions of the previous character	`go+gle`	`gogle`, `google`, `gooogle`
`?`	Matches zero or one repetition (makes preceding optional)	`colou?r`	`color`, `colour`
`\b`	Matches a word boundary	`\bcat\b`	`cat` but not `cats` or `cat.`
`\d`	Matches any digit (0-9)	`\d{}`	`123`, `456`, `007`
`\D`	Matches any non-digit character	`\D+`	`abc`, `!@#`
`\w`	Matches any alphanumeric character or underscore	`\w+`	`hello123`, `_var`
`\W`	Matches any non-word character	`\W+`	`!@#`, spaces
`\s`	Matches any whitespace character (space, tab, newline)	`\s+`	space, tab, newline
`\S`	Matches any non-whitespace character	`\S+`	`hello`, `123`

List of Functions:

re.match(pattern, string): searches for a match at the beginning of the string.

import re
text = "123abc"
dataObj = re.match(r"\d+", text)
if match:
    print(dataObj.group()) # Output: "123"
    print(dataObj.start()) # Output: 0,start position of matched string
    print(dataObj.end())   # Output: 3,end position of matched string

re.search(pattern, string): searches for a match anywhere in the string.

import re
text = "Order number: 12345"
dataObj = re.search(r"\d+", text)
print(dataObj.group())
# Output: "12345"

flags: re.I,re.M,re.S etc.

import re

text = """Apple
banana
cherry
apple
banana
fruit\\nList
"""

print(re.findall(r"apple", text, re.I))      # ['Apple', 'apple'] ignoring case
print(re.findall(r"^b", text, re.M))         # ['b', 'b']  multiple lines
print(re.findall(r"apple.*", text, re.S))    # ['apple\nbanana\nfruit\\nList\n'] ignoring newlines

re.findall(pattern, string): returns a list of all matches in the string.

import re
text = "apple 123, banana 456, cherry 789"
numbers = re.findall(r"\d+", text)
print(numbers)
# Output: ["123", "456", "789"]

re.finditer(pattern, string): returns an iterator over all matches in the string.

import r
text = "A1 B22 C333"
for dataObj in re.finditer(r"\d+", text):
    print(dataObj.group(), "at position", dataObj.start(), "-", dataObj.end())

# Output:
#1 at position 1 - 2
#22 at position 3 - 5
#333 at position 6 - 9

re..sub(pattern, replacement, string): replaces all matches with the replacement string.

import re
text = "I have 2 apples and 3 bananas"
new_text = re.sub(r"\d+", "#", text)
print(new_text)
# Output: "I have # apples and # bananas"

re.split(pattern, string): splits the string into a list of substrings based on the pattern.

import re
text = "word1, word2; word3:word4"
parts = re.split(r"[,;:]\s*", text)
print(parts)
# Output: ['word1', 'word2', 'word3', 'word4']

re.compile(pattern): creates a regex object you can reuse for multiple matches.r"..." = raw string (special characters not escaped by Python).

import re
pattern = re.compile(r"\d+")   # match one or more digits
result = pattern.findall("My number is 123 and pin is 4567")
print(result)
# Output: ['123', '4567']

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