Python Regular Expressions with the re Module: Syntax, Functions, and Practical Examples

Regular expressions are logical formulas for processing strings, using predefined special characters (metacharacters) and their combinations to define pattern strings that filter or match text. They are widely used in web crawling, data analysis, and string validation.

In Python, the re module provides full regex capabilities, offering both function-based and method-based interfaces that accept a pattern string as the first argument.

2 Basic Syntax

2.1 match function

The re.match(pattern, string, flags=0) function attempts to match the pattern only at the beginning of the string.

<code>re.match(pattern, string, flags = 0)</code>

Parameters:

pattern – the regular expression to match.

string – the target string to be searched.

flags – optional bitwise OR of flag constants that modify the matching behavior.

It returns a match object on success or None on failure; the match object’s group() or groups() methods retrieve the captured substrings.

Example (no match from the start):

<code>#未从初始位置匹配，会返回None
import re
line = 'i can speak good english'
matchObj = re.match(r'\s(\w*)\s(\w*).*', line)
if matchObj:
    print('matchObj.group() :', matchObj.group())
    print('matchObj.group(1) :', matchObj.group(1))
    print('matchObj.group(2) :', matchObj.group(2))
    print('matchObj.group(3) :', matchObj.group(3))
else:
    print('no match!')</code>

Example (match from the start):

<code>import re
line = 'i can speak good english'
matchObj = re.match(r'(i)\s(\w*)\s(\w*).*', line)
if matchObj:
    print('matchObj.group() :', matchObj.group())
    print('matchObj.group(1) :', matchObj.group(1))
    print('matchObj.group(2) :', matchObj.group(2))
    print('matchObj.group(3) :', matchObj.group(3))
else:
    print('no match!')</code>

2.2 search function

The re.search(pattern, string, flags=0) function scans the entire string and returns the first match found at any position.

<code>re.search(pattern, string, flags = 0)</code>

Parameters are the same as for match . It also returns a match object or None .

Example:

<code>import re
line = 'i can speak good english'
matchObj = re.search('(.*) (.*?) (.*)', line)
if matchObj:
    print('matchObj.group() :', matchObj.group())
    print('matchObj.group(1) :', matchObj.group(1))
    print('matchObj.group(2) :', matchObj.group(2))
    print('matchObj.group(3) :', matchObj.group(3))
else:
    print('no match!')</code>

2.3 sub function

The re.sub(pattern, repl, string, max=0) method replaces all occurrences of the pattern with repl (or up to max times) and returns the modified string.

<code>re.sub(pattern, repl, string, max=0)</code>

Example:

<code>import re
line = 'i can speak good english'
speak = re.sub(r'can','not', line)
print(speak)
speak1 = re.sub(r'\s','', line)  # remove all spaces
print(speak1)</code>

3 Special Regex Syntax

3.1 Character Classes

3.2 Special Character Classes

3.3 Repetition

3.4 Non‑greedy Repetition

3.5 Grouping with Parentheses

3.6 Backreferences

3.7 Anchors

3.8 Bracketed Special Syntax

For further reading, see the recommended articles on Python performance tips, decorators, data‑analysis tricks, and user‑profile implementation.