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Regular Expressions (Regex)

A Regular Expression (Regex) is a sequence of characters that forms a search pattern. It is used to check if a string contains a specified pattern, perform text search, and manipulate text.

Python has a built-in package called re to work with Regular Expressions.

1. Common re Functions

  • re.search(pattern, string): Returns a Match object if there is a match anywhere in the string.
  • re.findall(pattern, string): Returns a list containing all matches.
  • re.split(pattern, string): Returns a list where the string has been split at each match.
  • re.sub(pattern, replace, string): Replaces matches with a specified string.
import re

txt = "The rain in Spain"

# search()
x = re.search("rain", txt)
if x:
    print("Match found!")

# findall()
print(re.findall("ai", txt))  # Output: ['ai', 'ai']

2. Capture Groups

Parentheses () are used in regular expressions to define groups of characters. You can extract individual parts of a matched string using the group() method on a Match object.

  • group(0): Returns the entire matched string.
  • group(1): Returns the first parenthesized subgroup.
  • group(2): Returns the second parenthesized subgroup.
import re

text = "My phone number is 123-456-7890"
pattern = r"(\d{3})-(\d{3})-(\d{4})"  # Three capture groups

match = re.search(pattern, text)

if match:
    print(match.group(0))  # Full match: 123-456-7890
    print(match.group(1))  # Area code: 123
    print(match.group(2))  # Prefix: 456
    print(match.group(3))  # Line number: 7890

3. Reusing Patterns: re.compile()

If you plan to reuse the same regular expression pattern multiple times in your script, it is highly recommended to pre-compile the pattern using re.compile(). This optimizes performance and keeps your code cleaner.

import re

# Compile the pattern once
phone_pattern = re.compile(r"\d{3}-\d{3}-\d{4}")

lines = [
    "Call 123-456-7890 for support",
    "Or dial 987-654-3210 for sales"
]

# Use the compiled pattern
for line in lines:
    matches = phone_pattern.findall(line)
    print(matches)

4. Meta-characters Reference

  • []: A set of characters (e.g. [a-m]).
  • .: Any character except newline.
  • ^: Starts with (e.g. ^Hello).
  • $: Ends with (e.g. World$).
  • *: Zero or more occurrences.
  • +: One or more occurrences.
  • \d: Returns a match where the string contains digits (numbers 0-9).
  • \s: Returns a match where the string contains a white space character.
  • \w: Returns a match where the string contains word characters (alphanumeric and underscore).

Further Reading