Python RegEx

What a regular expression is

A regular expression (often shortened to regex) is a small language for describing patterns in text. Instead of asking whether a string equals an exact value, you describe its shape, for example three digits followed by a dash and four more digits, and let the engine find every part of the text that fits. Python ships regular expression support in the standard library, so you only need to import the re module before you can start matching.

import re

text = "The event starts at 10:30 today"
match = re.search(r"\d\d:\d\d", text)

if match:
    print("Found a time:", match.group())
else:
    print("No time found")

The core functions in re

Most day to day work uses a handful of functions from the module. Each one takes a pattern and the text to work on, and they differ mainly in what they return and how much of the string they consider.

Common metacharacters and quantifiers

Patterns are built from ordinary characters, which match themselves, and metacharacters, which carry special meaning. Combining these building blocks with quantifiers lets you describe how many times something may repeat.

• \d matches any digit, \w matches a letter, digit, or underscore, and \s matches whitespace.
• The dot . matches any single character except a newline by default.
• Square brackets such as [a-z] define a set of allowed characters.
• The quantifiers * (zero or more), + (one or more), and ? (zero or one) control repetition.
• Curly braces set an exact count, so \d{4} means exactly four digits.
• The anchors ^ and $ tie a pattern to the start or end of the string.

import re

log = "orders: A123, B456, C789"

# Find every code: one letter followed by three digits
codes = re.findall(r"[A-Z]\d{3}", log)
print(codes)  # ['A123', 'B456', 'C789']

# Parentheses create groups you can pull out separately
email = "contact jane.doe@example.com please"
m = re.search(r"(\w+\.\w+)@(\w+\.\w+)", email)
if m:
    print("user:", m.group(1))
    print("domain:", m.group(2))

Replacing text with re.sub()

Substitution is one of the most practical uses of regular expressions. You give re.sub() a pattern, a replacement, and the text, and it returns a fresh string with every match swapped out. This is handy for redacting sensitive data or normalising messy input.

import re

message = "Call 555-123-4567 or 555-987-6543"
hidden = re.sub(r"\d{3}-\d{3}-\d{4}", "[redacted]", message)
print(hidden)  # Call [redacted] or [redacted]