About Regex Cheatsheet

Character classes, quantifiers, anchors, groups, flags. Part of the DevTools Surf developer suite. Browse more tools in the Cheatsheets collection.

Use Cases

• Quickly looking up lookahead and lookbehind syntax differences
• Referencing quantifier behavior during log parsing scripts
• Finding the right character class shorthand for data validation
• Reviewing regex flag options when writing search-and-replace patterns

Tips

• Search by category: anchors, quantifiers, or character classes
• Review the flags section for global and case-insensitive options
• Use the groups reference when building complex capture patterns

Fun Facts

• Regular expressions were invented by mathematician Stephen Kleene in 1951 and first implemented in a computer by Ken Thompson in 1968.
• The POSIX standard defines two regex flavors: Basic Regular Expressions (BRE) and Extended Regular Expressions (ERE), which differ in metacharacter escaping.
• Catastrophic backtracking in regex can cause a single pattern to take exponential time — the Cloudflare outage on July 2, 2019, was caused by exactly this bug.

FAQ

Which flavor?

Common subset of PCRE, JavaScript, and Python. Language-specific features noted with flavor tags.

Named groups?

Yes — plus capture vs non-capture groups, backreferences, and how they work across languages.

Catastrophic backtracking?

Yes — the patterns to avoid (nested quantifiers, overlapping alternations) and safer alternatives.

Unicode character properties?

Covered — \p{L}, \p{N}, \p{Emoji}. Supported in modern JS (with u flag), Python re (with third-party regex module), PCRE natively.