Expresión Regular (Regex)Sintaxis
What is a regular expression (Regex)?
A structured sequence of characters defining a formal search pattern utilized by string-matching engines for parsing, validation, and replacement.
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Tester & Evaluator of Regular Expressions (Regex)
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//g
Test string
Step-by-step explanation
Type a regular expression to get started.
Copy-ready code
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const regex = //g;
const isMatch = regex.test(input);
const allMatches = [...input.matchAll(new RegExp(regex.source, regex.flags.includes("g") ? regex.flags : regex.flags + "g"))];Cheat Sheet — quick reference
Regex Engine Architecture & Pattern Matching
DFA vs NFA engines, ReDoS prevention, capture groups, and lookaround assertions.
NFA vs DFA Matching Engines
JavaScript and Python rely on NFA engines allowing backreferences and lookarounds, which introduces potential backtracking overhead compared to linear DFA engines.
Catastrophic Backtracking & ReDoS (O(2^n))
Nested quantifiers like `(a+)+$` cause exponential step explosions when failing near the end of strings, locking CPU threads in single-threaded environments.
Capture Groups vs Non-Capturing (?:...)
Non-capturing groups `(?:...)` cluster subexpressions without allocating match slots or capturing overhead, boosting throughput in high-volume parsing.
Zero-Width Lookarounds
Lookaheads and lookbehinds assert surrounding context without consuming characters, essential for multi-rule password policies and currency parsing.
Regex Performance, Escaping & Matching Troubleshooting
Step-by-step fixes for ReDoS locks, JavaScript lastIndex bugs, string escaping, and greedy matching.
Issue 1
CPU Spikes to 100% & UI Freezes (Catastrophic ReDoS)
⚡ Quick Fix:Refactor overlapping quantifiers `(a+)+` into disjoint atomic classes `[a-zA-Z]+`.
🔧 Technical Fix:Adopt timeout guards or linear O(n) engines like Google's RE2 for untrusted user-supplied regexes.
Issue 2
JavaScript `regex.test()` Alternates Between True and False
⚡ Quick Fix:Omit the `/g` flag when performing validation tests, or manually reset `regex.lastIndex = 0` before testing.
🔧 Technical Fix:Global RegExps mutate stateful `lastIndex` pointers; migrate to stateless `string.matchAll(regex)`.
Issue 3
Backslashes Stripped in Programming String Literals
⚡ Quick Fix:Use raw strings: prefix Python `r"\d+"`, C# `@"\d+"`, or double-escape `"\\d+"` in Java/JSON.
🔧 Technical Fix:Copy verified language-specific code snippets directly from the code generator tab in this tool.
Issue 4
Greedy Matching Consuming Multiple Tags/Lines
⚡ Quick Fix:Append `?` after quantifiers to enable lazy matching: `<.*?>` instead of greedy `<.*>`.
🔧 Technical Fix:Prefer negated character classes like `<[^>]*>` over lazy quantifiers for higher throughput without backtracking.
Did You Know? Regex History & Trivia
Zawinski's iconic quote, the Unix grep origin, and 1951 neural network foundations.
💬
Jamie Zawinski's Legendary 1997 Quote
"Some people, when confronted with a problem, think 'I know, I'll use regular expressions.' Now they have two problems." Became software engineering's most famous quip.
🧠
Invented in 1951 to Model Biological Neural Circuits
Mathematician Stephen Kleene invented regular algebra to mathematically describe firing patterns in biological neural circuits.
💻
How the Unix 'grep' Command Got Its Name
Ken Thompson added regex to Unix `ed`. The command `g/re/p` (Global Regular Expression Print) became the standalone utility `grep`.
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PCRE: The C Engine Powering the Global Web
Written by Philip Hazel in 1997, PCRE powers Apache, Nginx, PHP, and hundreds of mission-critical systems worldwide.
Regular Expressions & Regex Engines Glossary
Learn the fundamental concepts, protocols, and technical terminology of this tool.
Retroceso Catastrófico (ReDoS)Seguridad
What is Catastrophic Backtracking (ReDoS)?
An algorithmic security flaw where ambiguous nested quantifiers trigger exponential evaluation loops, freezing host CPU threads in denial-of-service.
Grupos de CapturaParsing
What are regex capture groups?
Subpatterns enclosed in parentheses `(...)` isolating matched token substrings for programmatic extraction and backreference replacement.
Lookahead / LookbehindAvanzado
What are Lookaround regex assertions?
Zero-width pattern assertions matching a character sequence only if followed or preceded by another specific pattern without consuming string length.