Write, test and share regular expressions in real time. Paste a pattern, see matches highlighted instantly, grab ready-made code snippets, and let Codeground AI explain every capture group — no sign-up needed.
g global · i case insensitive · m multiline · s dotAllAI reads your pattern, flags, and match list, then explains every group and token in plain English using JavaScript RegExp semantics.
Codeground AI's Regex Tester is a free, browser-based tool that lets you write, debug, and share JavaScript regular expressions in real time. Paste your pattern, type or upload a test string, and see every match highlighted on the fly — no installation, no account required.
Regular expressions (regex) are one of the most powerful tools in a developer's toolkit. They are used for form validation, text parsing, log analysis, search-and-replace in code editors, data extraction pipelines, and much more. Our tool removes the guesswork so you can build correct patterns faster.
Matches update as you type — no "Run" button needed. See exactly which characters are captured, how many times, and where in the string.
Every match is highlighted inline in your test string using a distinct green background, making it trivial to spot partial vs. full matches.
Hit Explain pattern & matches to have Codeground AI walk you through every token, flag, and capturing group in plain English.
Copy a full share URL (pattern + test string encoded in the URL) or generate a compact short link that stays live for ~90 days.
One click copies production-ready regex usage in JavaScript, Python, Java, or Go — including the correct flags for each language.
Choose from a curated library of common patterns — email, URL, phone, IP address, dates, hex colors, and more — loaded with a sample string.
g, i, m, s) in the small flag box to the right. The tester uses the JavaScript built-in RegExp engine — the same engine running in Chrome, Firefox, Node.js, and Deno. Results are 100% accurate for any JS application.
Yes. JavaScript RegExp does not support certain PCRE features like named back-references in all contexts, possessive quantifiers, or (?P<name>) Python-style group syntax. The Code Snippets panel adapts the pattern for Python, Java, and Go accordingly.
Escape it with a backslash: \. matches a literal dot, \* matches a literal asterisk. The rule is: any character that has a special regex meaning (. * + ? ^ $ { } [ ] ( ) | \) must be preceded by \ to be treated as a literal.
By default, quantifiers like + and * are greedy — they match as much as possible. Adding ? after a quantifier makes it lazy (+?, *?), matching as little as possible. Example: against <a><b>, the pattern <.+> matches the whole string, while <.+?> matches only <a>.
g flag do and when should I leave it off? The g (global) flag makes the engine find all non-overlapping matches in the string. Without it, only the first match is returned. Leave off g when you only need to check whether a pattern exists (regex.test(str)) or when you use String.replace and only want to replace once.
A capture group(pattern) saves the matched text so you can reference it later (e.g., in a replace string as $1, or via match[1]). A non-capturing group(?:pattern) groups the pattern for quantifiers or alternation without storing the result — useful when you need grouping but not the overhead of a back-reference.
Matching, highlighting, and code-snippet generation all run entirely in your browser — no data leaves your machine. The only time a server call is made is when you click Explain with AI (the pattern and test string are sent to Codeground AI) or when you create a Short link (the content is stored on Codeground AI servers for ~90 days).
Catastrophic backtracking occurs when a pattern with nested quantifiers (e.g., (a+)+) causes the engine to explore an exponential number of possible paths before deciding there is no match. Avoid it by: using possessive quantifiers or atomic groups (not supported in JS), rewriting ambiguous alternations, or using a specific instead of an overly general pattern. The AI explain feature can spot likely problem patterns.
Yes — completely free, no sign-up required. The tool, AI explanations, and short links are provided by Codeground AI, the online IDE and developer tools platform at codeground.ai.