Regular expressions: a practical cheat sheet

By Uxair Ijaz · 2026-10-22 · 2 min read

Regular expressions look cryptic, but a small set of pieces covers most real work. Keep this page open next to the regex tester and try each pattern as you read.

Characters

Pattern	Matches
`.`	any character except a newline
`\d` `\w` `\s`	a digit, a word character (letter, digit, `_`), whitespace
`\D` `\W` `\S`	anything except those
`[abc]`	one of a, b or c
`[^abc]`	any character except a, b or c
`[a-z0-9]`	a range
`\.`	a literal dot (escape special characters with `\`)

How many

Pattern	Means
`*`	0 or more
`+`	1 or more
`?`	0 or 1 (optional)
`{3}`	exactly 3
`{2,5}`	2 to 5
`*?` `+?`	as few as possible (lazy)

Quantifiers are greedy by default: <.+> on <b>bold</b> matches the whole string. <.+?> matches just <b>.

Where

Pattern	Means
`^` `$`	start and end of the text (or of each line with the `m` flag)
`\b`	a word boundary
`(?=…)` `(?!…)`	followed by / not followed by (lookahead)
`(?<=…)` `(?<!…)`	preceded by / not preceded by (lookbehind)

Groups

Pattern	Means
`(…)`	capture group, reused as `$1` in replacements
`(?<year>…)`	named group, `lt;year>`
`(?:…)`	group without capturing
`a\|b`	a or b

Flags

g find all matches · i ignore case · m ^ and $ per line · s dot matches newlines · u full Unicode

Patterns you can copy

Email (practical)      ^[^\s@]+@[^\s@]+\.[^\s@]{2,}$
URL                    https?://[^\s/$.?#].[^\s]*
ISO date               \b\d{4}-\d{2}-\d{2}\b
Time 24h               \b([01]\d|2[0-3]):[0-5]\d\b
Hex colour             #(?:[0-9a-fA-F]{3}){1,2}\b
Whole word "cat"       \bcat\b
Trim spaces            ^\s+|\s+$
Repeated word          \b(\w+)\s+\1\b

For email, a simple pattern plus a confirmation email beats a "perfect" 6,000-character regex.

Tips

Start small and grow the pattern while watching the matches highlight.
Anchor with ^…$ when validating a whole value, otherwise \d+ happily matches inside abc123.
Beware catastrophic backtracking: nested quantifiers like (a+)+$ can freeze on long inputs.
Use named groups in anything you will read again in six months.