Regex Cookbook Cheat Sheet

A practical reference of regular expression syntax, character classes, quantifiers, groups, and common validation patterns.

2 PagesIntermediateFeb 12, 2026

Character Classes & Anchors

Building blocks for matching character types and positions.

.- Matches any character except newline (unless dotall/s flag is set)
\d \w \s- Digit, word character (`[A-Za-z0-9_]`), whitespace — uppercase negates each
[abc] / [^abc]- Matches any character in the set / any character NOT in the set
[a-z0-9]- Character range within a set
^ / $- Start of string (or line in multiline mode) / end of string or line
\b / \B- Word boundary / non-word boundary

Quantifiers & Groups

Repetition and capturing syntax.

* + ?- Zero or more, one or more, zero or one occurrences
{n,m}- Between n and m repetitions; `{n}` exact, `{n,}` at least n
(...)- Capturing group; can be referenced later as \1, $1, etc.
(?:...)- Non-capturing group; groups without creating a backreference
(?<name>...)- Named capturing group, referenced as group('name') in most engines
*? +? ??- Lazy (non-greedy) versions of the quantifiers, matching as little as possible

Common Validation Patterns

Ready-to-use patterns for typical inputs.

regex

^[\w.+-]+@[\w-]+\.[a-zA-Z]{2,}$        # basic email^https?://[\w.-]+(:\d+)?(/\S*)?$        # basic URL^\d{3}-\d{3}-\d{4}$                       # US phone (123-456-7890)^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$      # password: upper+lower+digit, 8+ chars^\d{4}-\d{2}-\d{2}$                          # ISO date YYYY-MM-DD^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$             # hex color code

Lookaround & Flags

Zero-width assertions and common engine flags.

regex

foo(?=bar)     # lookahead: 'foo' only if followed by 'bar'foo(?!bar)     # negative lookahead: 'foo' NOT followed by 'bar'(?<=foo)bar    # lookbehind: 'bar' only if preceded by 'foo'(?<!foo)bar    # negative lookbehind: 'bar' NOT preceded by 'foo'# Common flags/pattern/i     # case-insensitive/pattern/g     # global (find all matches)/pattern/m     # multiline (^ and $ match per line)/pattern/s     # dotall (. matches newline too)

Pro Tip

Avoid nested quantifiers like `(a+)+` on untrusted input — they can cause catastrophic backtracking (ReDoS); prefer possessive quantifiers, atomic groups, or a non-backtracking engine when the pattern must run on user-supplied data.

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