Master Python Regular Expressions: Patterns, Methods, and Real‑World Examples

Python Regular Expressions

Regular expressions are special character sequences that let you test whether a string matches a particular pattern. Since Python 1.5, the re module provides Perl‑style regex support, giving Python full regex capabilities.

Regex Pattern Syntax

Pattern strings use a special syntax. Literal letters and digits match themselves, while escaped characters gain special meanings. Metacharacters include: ^ – start of string $ – end of string . – any character (except newline, unless re.DOTALL is set) [...] – character class, e.g., [amk] matches a, m or

k

[^...]

– negated character class re* – zero or more repetitions re+ – one or more repetitions re? – optional (non‑greedy) re{n} – exactly n repetitions re{n,} – n or more repetitions re{n,m} – between n and m repetitions (greedy) a|b – alternation (re) – grouping (?imx) – inline flags (ignore case, multiline, verbose) (?-imx) – turn off flags (?:re) – non‑capturing group (?=re) – positive look‑ahead (?!re) – negative look‑ahead \w – word character (letters, digits, underscore) \W – non‑word character \s – whitespace \S – non‑whitespace \d – digit \D – non‑digit \b – word boundary \B – non‑word boundary

re Module Overview

The re module compiles a pattern into a regex object, which can then be used for matching and substitution. Compiling once improves performance compared with using the module‑level functions directly.

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# @Time    : 2018/4/29 22:02
# @Author  : Feng Xiaoqing
# @File    : test.py
# @Function: -----------
import re
import timeit

print(timeit.timeit(setup='''import re; reg = re.compile('<(?P<tagname>\w*)>.*</(?P=tagname)>')''', stmt='''reg.match('<h1>xxx</h1>')''', number=1000000))
print(timeit.timeit(setup='''import re''', stmt='''re.match('<(?P<tagname>\w*)>.*</(?P=tagname)>', '<h1>xxx</h1>')''', number=1000000))

reg = re.compile('<(?P<tagname>\w*)>.*</(?P=tagname)>')
reg.match('<h1>xxx</h1>')

Typical output shows the compiled version is faster:

0.4229613832757271 1.0246964437151256

re.compile(pattern[, flags])

Common flags: re.I – ignore case re.M – multiline (changes ^ and $) re.S – dot matches newline re.L – locale‑dependent character classes re.U – Unicode‑aware character classes re.X – verbose mode (allows whitespace and comments)

Example:

import re
pattern = re.compile(r'\d+')  # match one or more digits
m = pattern.match('one12twothree34four')
print(m)               # None – no match at start
m = pattern.match('one12twothree34four', 3, 10)
print(m)               # <_sre.SRE_Match object ...>
print(m.group())       # '12'
print(m.start())       # 3
print(m.end())         # 5
print(m.span())        # (3, 5)

re.match

re.match(pattern, string, flags=0)

attempts to match at the beginning of the string. Returns a match object or None.

re.search

re.search(pattern, string, flags=0)

scans the whole string and returns the first match.

re.findall

Returns a list of all non‑overlapping matches:

import re
pattern = re.compile(r'\d+')
print(pattern.findall('runoob 123 google 456'))  # ['123', '456']
print(pattern.findall('run88oob123google456', 0, 10))  # ['88', '12']

re.finditer

Returns an iterator yielding match objects:

import re
for m in re.finditer(r"\d+", "12a32bc43jf3"):
    print(m.group())
# 12
# 32
# 43
# 3

re.split

Splits a string by the occurrences of a pattern:

import re
print(re.split(r'\W+', 'runoob, runoob, runoob.'))
# ['runoob', 'runoob', 'runoob', '']
print(re.split(r'(\W+)', ' runoob, runoob, runoob.'))
# ['', ' ', 'runoob', ', ', 'runoob', ', ', 'runoob', '.', '']

re.sub

Replaces matches with a replacement string or function:

import re
phone = "2004-959-559 # 这是一个国外电话号码"
num = re.sub(r'#.*$', "", phone)
print("电话号码是:", num)          # 2004-959-559 
num = re.sub(r'\D', "", phone)
print("电话号码是 :", num)          # 2004959559

Key Takeaways

Use re.compile to create a regex object for repeated operations, choose appropriate flags for case‑insensitivity or multiline handling, and select the right function ( match, search, findall, finditer, split, sub) based on the task.