Cómo usar esta calculadora
Building a pattern the way the engine sees it
A regular expression is a miniature program: the engine walks your text, and at every position it asks whether the pattern matches starting there. The tester makes that walk visible — every match is listed with its position, the exact text it matched, and the capture groups it produced, updated as you type. Start with the literal that must appear (request_id=, say), add the flexible parts around it (a w+ for the value), and check the result against real lines from your data before you trust it anywhere.
The engine here is the platform’s ECMAScript RegExp — the same one JavaScript, Node.js, Deno, and every modern browser use. That is the point of the tool’s fidelity promise: a pattern that matches here matches in your code, and one that fails here will fail there. Dialects like PCRE (PHP, grep, regex101’s default) have features ECMAScript lacks or spells differently — lookbehind syntax, named-group conventions, atomic groups — and the tester rejects them with the platform’s error message rather than pretending to support a dialect it does not run.
Using the flags deliberately
Flags are not decorations — they change what the pattern means. Without g, the engine stops at the first match, and the classic "why does my pattern match once?" confusion is usually a missing g. Without m, the anchors ^ and $ bind to the whole string, not to line boundaries, so a pattern that should match each log line silently matches nothing when the text has multiple lines. The s flag is the one that surprises everyone: . matches every character except line breaks unless s is set, so patterns that should span lines fail on the newline. And the difference between the unicode u and non-unicode modes changes how astral characters like emoji are treated — a point where the ECMAScript engine is stricter than older dialects.
The tester shows the flags you have set at a glance and re-runs the scan on every change, so each flag’s effect is visible the moment you toggle it. That is the fastest way to learn what a flag does: switch it, watch the match list change.
Reading matches, groups, and failures
Each result row shows the matched text, its start position, and its capture groups — the parts of the pattern inside parentheses, which are the values your code will actually consume. The classic use is extraction: with /request_id=(w+)/g against a log line, group 1 holds the id, and the tester shows both matches and their group values before you wire the pattern into a pipeline.
A pattern that matches nothing is a bug report, not a puzzle: the tool shows every match position so the failure is visible at a glance — the pattern matched a different place than you expected, or matched zero times because of a missing flag, a literal space, or an anchor bound to the wrong position. Malformed patterns return the platform’s own error message, which names the position of the problem, so fixing is a matter of reading the message rather than guessing.
Safety and the honest limits
Regular expressions can be pathological: certain patterns take exponential time on certain inputs — the classic "catastrophic backtracking" failure mode — and the tester caps match scanning so a hostile pattern cannot freeze the page. The cap is disclosed; extremely large texts or pathological patterns are truncated with a notice rather than run to completion.
Two things the tool deliberately does not do: it does not judge whether a pattern is efficient enough for production traffic (that is a load-testing question), and it does not claim your pattern is correct in the abstract — it shows what the engine does with your text, which is the only honest answer. And because patterns and pasted text often contain credentials and personal data, everything runs locally: nothing is uploaded, logged, or stored.
Preguntas frecuentes
¿Qué dialecto de regex usa esta herramienta?
El motor ECMAScript de la plataforma: el mismo dialecto que usan JavaScript, Node.js, Deno y los navegadores. Dialectos como PCRE (PHP, grep, el predeterminado de regex101) tienen características extra, como lookbehind en formas distintas y grupos con nombre con sintaxis diferente; esas no forman parte de ECMAScript.
¿Por qué mi patrón no ha coincidido con nada?
Causas habituales: falta el flag g cuando esperas varias coincidencias, olvidar que los anclajes como ^ y $ solo coinciden con límites de línea con el flag m, y patrones que incluyen espacios literales sin querer. La herramienta muestra cada coincidencia con su posición, así que el fallo suele verse de un vistazo.
¿Mi texto se envía a algún sitio?
No. El patrón y el texto se procesan íntegramente en la pestaña de tu navegador: nada se sube, registra ni almacena. Esto importa con fragmentos de logs y archivos de configuración que a menudo contienen credenciales o datos personales.
What does the u (unicode) flag actually change?
It switches the engine to strict Unicode mode: astral characters (emoji, rare scripts) are treated as single code points instead of surrogate halves, and some otherwise-lenient syntax becomes an error. Patterns that work without u can behave differently with it — the tester makes the difference visible as you toggle.
Can I use the same pattern in Python or Go?
Not safely — those languages use different engines (Python’s re, Go’s RE2). The fidelity promise here is specifically ECMAScript: what matches here matches in JavaScript, Node.js, Deno, and browsers. Porting a pattern to another language means re-testing it in that language’s tester.