Regular Expressions Cheat Sheet - CodeSwordsman

Dot matches any char except newline; \d digit, \w word char, \s whitespace

Uppercase forms negate: non-digit, non-word, non-whitespace

Zero or more, one or more, and zero or one

Bounded repetition: {3} exactly three, {3,} at least three

Appending ? makes it lazy, stopping at the nearest closing token

Character classes select one option; ^ inside brackets negates

^ anchors the start and $ the end; use the m flag for multiline text

Word boundary prevents matching word inside sword

Parentheses capture groups referenced as $1 and $2 in replacements

Non-capturing group clusters alternatives without numbering

A backreference requires the same text again, handy for duplicate words

Positive lookahead matches digits followed by a unit without consuming it

Positive lookbehind matches digits preceded by a currency symbol

Negative lookahead excludes matches followed by a specific unit

Global match returns all results instead of the first one

Case-insensitive matching instead of writing [a-zA-Z]

m makes ^ and $ per line, s lets the dot match newlines

The u flag enables Unicode property escapes such as Han script

Basic email validation; still verify by sending a confirmation mail

Permissive URL check that blocks obviously invalid input

IPv4 validation constraining each octet to 0-255

YYYY-MM-DD format check without month-length validation

-P enables PCRE for \d and lookarounds; BSD/macOS grep lacks it

-E enables extended regex so | and + need no escaping

Escape metacharacters when replacing; double backslashes in RegExp strings