Regex Cheat Sheet for Beginners

A regular expression, or regex, is a pattern that describes text. You use it to find, check or replace pieces of a string. It looks like line noise at first, but a handful of building blocks do most of the work.

Matching characters

• . matches any one character (except a line break).
• \d matches a digit, \w a letter, digit or underscore, \s a space or tab. Capital versions (\D, \W, \S) mean the opposite.
• [abc] matches a, b or c. [a-z] matches any lowercase letter. [^abc] matches anything except a, b or c.
• \. matches a real dot. Put a backslash before any special character to match it literally.

Repeating

• * means zero or more, + means one or more, ? means zero or one.
• {3} means exactly 3, {2,4} means 2 to 4, {2,} means 2 or more.
• Add ? after a quantifier (.*?) to make it lazy, so it matches as little as possible instead of as much as possible.

Position and choice

• ^ matches the start and $ the end of the text (or of each line with the m flag).
• \b matches a word boundary, so \bcat\b finds "cat" but not "concatenate".
• a|b matches a or b.

Groups

• (...) groups part of the pattern and captures what it matched, available as $1, $2 and so on.
• (?<year>\d{4}) gives a group a name, so you can use $<year> in a replacement.
• (?:...) groups without capturing.
• (?=...) and (?!...) look ahead without using up characters.

Flags

• g finds all matches, not just the first.
• i ignores upper and lower case.
• m makes ^ and $ work on every line.
• s lets . match line breaks.
• u turns on full Unicode support.

Examples

• ^\d{4}-\d{2}-\d{2}$ matches a date like 2026-10-16.
• #[0-9a-fA-F]{6}\b matches a hex color like #6366f1.
• \b[\w.+-]+@[\w-]+\.[\w.]+\b finds simple email addresses. (Perfect email validation with regex isn't practical, so use this only to find them.)

Watch out for slow patterns

Patterns with nested repeats, such as (a+)+$, can take a very long time on certain text. This is called catastrophic backtracking. If a pattern feels slow, make it more specific.

Practice with the Regex Tester: it highlights every match and shows the capture groups as you type.