Regular Expressions

Regex Groups and Backreferences

These notes cover how to use groups and backreferences in Regular Expressions within Python.

The core idea is to create groups within a regex pattern to capture specific parts of the matched string. These captured parts can then be reused (backreferenced) within the same pattern or accessed after the match is made.

Creating Groups:

Groups are created using parentheses () in your regular expression. Each opening parenthesis starts a new group, and the corresponding closing parenthesis ends it.

import re

text = "My phone number is 123-456-7890 and my zip code is 90210"
pattern = r"(\d{3})-(\d{3})-(\d{4})"  # Three groups: area code, prefix, line number

match = re.search(pattern, text)
if match:
    print(match.group(0))  # Entire match
    print(match.group(1))  # First group (area code)
    print(match.group(2))  # Second group (prefix)
    print(match.group(3))  # Third group (line number)

Backreferences:

Backreferences allow you to refer to previously captured groups within the same regular expression. This is done using backslash followed by the group number. \1 refers to the first group, \2 to the second, and so on.

text = "abababa"
pattern = r"(a)(b)\1\2\1" # Matches "abababa"  \1 refers to (a), \2 refers to (b)

match = re.search(pattern, text)
if match:
    print(match.group(0))

Named Capture Groups:

Python also supports named capture groups, making your code more readable. These are defined using the (?P<name>...) syntax.

text = "My email is test@example.com"
pattern = r"(?P<user>[^@]]+)@(?P<domain>[^@]]+)"

match = re.search(pattern, text)
if match:
    print(match.group("user"))  # Access using name
    print(match.group("domain"))

Regex Metacharacters (Python’s re Module)

This is useful for extracting specific parts of a matched string in a structured way. Refer to Regex Metacharacters for more information on the basic building blocks of Regular Expressions used in these examples. The Python’s re Module note will cover the different functions available within Python’s re module for working with Regular Expressions.