Regex Cheat Sheet for Everyday Code

Character classes

Pattern	Matches
\d	a digit (0-9)
\w	a word character (A-Za-z0-9_)
\s	whitespace
.	any char except newline (with s flag: any char)
[abc]	any one of a, b, c
[^abc]	anything except those

Quantifiers

Pattern	Meaning
a*	zero or more
a+	one or more
a?	zero or one
a{2,4}	between two and four
a{2,}	two or more

Groups & alternation

• (abc) — capture group ($1, $2… in replacement).
• (?:abc) — non-capturing group.
• a|b — alternation ("a or b").
• (?<name>…) — named group.

Anchors & boundaries

• ^ start of string (with m: start of line).
• $ end of string (with m: end of line).
• \b word boundary — \bcat\b matches "cat" but not "category".

Flags

• g — global (find all, needed for matchAll).
• i — case-insensitive.
• m — multiline (^/$ act per line).
• s — dot matches newlines.
• u — unicode mode (enables \p{…}).

Everyday patterns

Job	Pattern
Email (practical)	^[^\s@]+@[^\s@]+\.[^\s@]+$
Date YYYY-MM-DD	^\d{4}-\d{2}-\d{2}$
Slug	^[a-z0-9]+(?:-[a-z0-9]+)*$
Hex color	^#?[0-9a-fA-F]{6}$
Leading/trailing space	`^\s+
Extract numbers	\d+ with g

Replacing with groups

"2026-09-09".replace(/(\d{4})-(\d{2})-(\d{2})/, "$3/$2/$1")
// "09/09/2026"

Greedy vs lazy

Quantifiers are greedy by default: a.+b against a1b2b matches the whole a1b2b, taking the last possible b. Add ? to make it lazy — a.+?b matches the shortest, a1b. When you find a pattern matching too much, suspect greed before suspecting your character class.

Common mistakes

• Forgetting the g flag. "aaa".match(/a/) returns one match; with g it returns all three. Tools and languages often behave differently here.
• Unescaped dots. . matches any character. To match a literal dot in a domain or file name, write \..
• Anchors that are too loose. /\d{4}/ matches anywhere in the string. Wrap with ^…$ for a full-match check.
• Catastrophic backtracking. Nested quantifiers like (a+)+ on long strings can freeze the page. Keep nesting shallow, and test on worst-case input in the regex tester before deploying it in validation hot paths.

Practice it

Test every pattern above in ToolsKit's regex tester before you trust it in code. Build the pattern against the real samples you will validate, not just the happy path — one tricky input is worth ten clean ones.