Hoja de Referencia Regex

Regex Cheat Sheet — Complete Regular Expression Reference with Ready-to-Use Patterns

Regular expressions are one of the highest-leverage skills in software development — a single well-written regex can replace 50 lines of string parsing code. Yet most developers use only a small fraction of what regex can do, defaulting to basic patterns while complex validation, extraction, and transformation tasks pile up. This cheat sheet covers the full regex toolkit: anchors, character classes, quantifiers, groups, lookarounds, flags, and a library of ready-to-use patterns for common validation tasks including Indian-specific formats like PAN and GSTIN.

How to use this cheat sheet

Scan the section that matches your problem — anchor, character class, quantifier, group, or lookaround. Copy the pattern, then test it live in our Regex Tester to verify it against your actual input. The patterns in this reference work in JavaScript and PCRE flavors. Where there are differences between flavors (like lookbehind variable-length support or possessive quantifiers), the notes column calls it out. The "Common ready-to-use patterns" section at the bottom is a direct copy-paste library — email validation, URL matching, IPv4, Indian phone numbers, UUID, ISO dates, hex colours, URL slugs, PAN, and GSTIN patterns are all tested and ready to use.

The four pillars of regex you must know

Anchors (^ $ \b) control where matches are allowed to occur in the string — without anchors, a pattern matches anywhere in the input including partial matches. Character classes (\d \w \s . [abc] [^abc]) define what characters are acceptable at each position. Quantifiers (* + ? {n,m} and their lazy variants) control how many times a character or group can repeat. Groups and alternation ((abc) (?:abc) a|b) allow you to match sequences as units, capture substrings for extraction, and provide multiple valid pattern alternatives. Everything else in regex — lookarounds, backreferences, flags — is built on these four foundations. Master these and you can construct any regex you need.

Who uses this cheat sheet

Backend developers validating form input use it to find or verify patterns for email, phone, postal code, and ID format validation. Data engineers writing ETL pipelines use it for text extraction and transformation patterns on log data, CSV files, and API responses. Security researchers use it when writing WAF rules or analysing log patterns for intrusion detection. Front-end developers building form validation use the ready-to-use patterns section as a starting point and customise from there. Developers working on Indian applications specifically use it for PAN number and GSTIN validation patterns that are hard to find correctly written elsewhere.

Privacy and data handling

This is a static reference page — no data is collected or transmitted. Test patterns against your own data using the linked Regex Tester, which also runs entirely in your browser.