Regex Tester
Test JavaScript regular expressions against sample text, flags, capture groups, escaping rules, and release-review examples before copying a pattern.
- Check positive cases, negative cases, edge cases, flags, escaping, and capture groups.
- Use realistic sample text instead of a single happy-path example.
- Watch for broad wildcards, nested quantifiers, and patterns that may run slowly on long input.
Static fallback example
The live Regex Tester workbench runs in the browser. If it does not load, use this static example to confirm the expected input and output pattern, then enable JavaScript or reload the page to run the tool.
The live tester compiles JavaScript regular expressions in the browser and caps long match output to keep the page usable. The static example only shows the pattern/text relationship.
About this tool
The Regex Tester checks regular expression patterns against sample text so matches, groups, and flags can be reviewed before use.
When this tool is useful
Use it to test validation patterns, log filters, text extraction rules, replacement drafts, and documentation examples with realistic sample input.
A pattern that works on one sample may still fail on real data. Test positive cases, negative cases, and edge cases.
Input and output example
Pattern: \b\w+@\w+\.com\b
Sample text with an email-like value should show the matched substring and make escaping mistakes easier to find.
Performance caution
Nested quantifiers and broad wildcards can cause slow matching on long text. Keep patterns specific when they will run on production input.
Always escape user-provided text before embedding it inside a regular expression.
Before copying the result
Keep the final pattern, flags, positive samples, negative samples, and edge cases beside the match output. A regex that passes one sample can still fail on punctuation, Unicode text, multiline input, or unexpected separators.
Review escaping in the language or platform that will actually run the expression. Avoid broad wildcards and nested quantifiers on untrusted or long input, and validate production behavior outside this quick tester.
FAQ
Why does a backslash disappear?
Some programming languages require string escaping before the regex engine sees the pattern.
Can one regex validate every email address?
Not reliably. Use a practical pattern for UI checks and server-side validation for real workflows.
JavaScript regex testing workflow and safety checks
Regular expressions are powerful for validation, extraction, and cleanup, but small pattern mistakes can silently match too much, too little, or run slowly on long input. A regex tester should be used with representative examples, negative cases, and clear notes about the JavaScript regex engine.
Recommended workflow
- Start with three examples that should match and three that should not match.
- Add anchors only when the entire string must match. Without ^ and $, a pattern may pass because one substring matches.
- Check flags such as g, i, m, s, and u deliberately; flags change how matches are found.
- After the pattern works in the tester, copy it into the application with escaping rules appropriate for the target language or string literal.
Example review
Input: Pattern: ^[A-Z]{2}-\d{4}$, sample: AB-1234
What to verify: The sample should match, while AB-12345, ab-1234, and XX_1234 should fail.
Common mistakes to check
- Using a positive example only and never testing near-misses.
- Forgetting to escape backslashes when moving from the tester to a JavaScript string.
- Writing catastrophic patterns for long untrusted input.
- Using regex for full parsing tasks better handled by a parser.
Copying and verification standard
Before copying output from this page, compare the result with the original input and write down the reason the transformed value is acceptable. For developer utilities, this usually means checking field count, escaped characters, casing, whitespace, time unit, or syntax boundary rather than trusting that a visible result is automatically correct.
For repeatable work, save a short non-sensitive sample and the expected result. That gives you a quick regression example when a browser extension, copied rich text, cached script, or upstream data source changes the behavior you observe.
Operational FAQ
Why does my pattern work here but not in code?
The pattern may need extra escaping inside a string literal, or the runtime may use a different regex engine.
Can regex validate every email address perfectly?
No practical email regex is perfect for every valid address. Use regex for basic screening and server-side validation for real workflows.
Review standards by context
The same tool can support several workflows, but each workflow needs a different acceptance check. Use the following context notes to decide whether the output is ready to copy or whether the source material needs another pass.
- Form validation: Add near-miss values and confirm the pattern rejects them, not only that it accepts one valid example.
- Extraction: Test repeated matches, optional groups, and empty captures before applying the pattern to long text.
- Code migration: Recheck escaping when moving between a raw regex literal, a JavaScript string, JSON, and another programming language.
When a value will be used in production documentation, an incident report, a customer reply, or a configuration change, keep the source value and the transformed value together. That record makes the tool result auditable instead of leaving only a copied output with no explanation.
A complete regex review also checks runtime context. The tester can show matches on sample text, but it cannot guarantee production performance, prevent catastrophic backtracking, sanitize user input, or prove the pattern handles every real-world case.
JavaScript regex testing workflow for extraction, validation, and release review
A regex tester is useful only when the sample text represents the real input stream. A pattern that works on one short example can fail on multiline text, Unicode characters, repeated groups, escaped delimiters, or unexpectedly long strings. This page should help users test a pattern as an operational review, not as a one-line trick.
Use this page for JavaScript regular expressions. Syntax and behavior can differ from PCRE, .NET, RE2, database regex engines, and command-line tools. Before copying a pattern into production, confirm flags, anchors, capture groups, replacement behavior, and worst-case performance in the target runtime.
Use the tool as a review workflow
- Write one positive sample, one negative sample, and one boundary sample before changing the pattern.
- Select flags deliberately. The g flag changes repeated matching, i changes case sensitivity, m changes line anchors, s changes dot behavior when supported, and u changes Unicode handling.
- Check anchors against the target input. ^ and $ behave differently with multiline mode; word boundaries can surprise users with non-English text.
- Review capture groups by name or index. A later edit can shift group numbers and break replacement code.
- Before production use, test long input and repeated near-matches to avoid catastrophic backtracking.
Three practical examples
| Scenario | Sample input | What to check before copying |
|---|---|---|
| Extract order IDs | Order ID: ORD-2026-00041 | Use a bounded pattern such as ORD-\d{4}-\d{5} and verify that it rejects partial IDs. |
| Validate simple slug | release-notes-2026-06 | Anchor the pattern from start to end and decide whether uppercase, underscores, or trailing hyphens are allowed. |
| Multiline log search | ERROR first line\nINFO next line | Enable or avoid multiline mode deliberately depending on whether ^ should match each line or only the whole string. |
Common error table
| Symptom | Likely cause | How to confirm | Practical fix |
|---|---|---|---|
| Pattern matches too much text | Greedy quantifier such as .* expands until the last possible delimiter. | Compare the first and last match boundary in a sample containing two delimiters. | Use a lazy quantifier, a negated character class, or a more specific delimiter rule. |
| Pattern works in another tester but fails here | The source tester used PCRE, RE2, or a different feature set than JavaScript. | Check lookbehind, named group, flag, and escape support in the target environment. | Rewrite the pattern for JavaScript or test in the final runtime. |
| No match at line boundaries | The m flag was missing or anchors were used for the wrong scope. | Test the same text with and without m and inspect where ^ and $ apply. | Use m for per-line anchors, or use explicit newline handling. |
| Replacement inserts the wrong group | A new capturing group changed numeric group positions. | Print each captured group for the sample text. | Use non-capturing groups for structure and named groups when the runtime supports them. |
| Browser tab freezes on long input | Nested quantifiers or ambiguous alternation produce catastrophic backtracking. | Test a shorter near-match and watch runtime grow sharply with length. | Constrain quantifiers, remove nested ambiguity, or switch to a linear parser for complex input. |
Security and privacy boundary
Regex samples frequently come from logs, emails, access paths, customer messages, or production exports. Patterns can be public while samples are not.
- Redact names, email addresses, IP addresses, account IDs, and tokens before using production log excerpts.
- Keep enough structure to reproduce the match: delimiters, line breaks, and representative Unicode characters matter.
- Do not paste entire customer messages when a two-line synthetic sample would exercise the same pattern.
- When sharing a pattern, include sample inputs and expected matches so reviewers do not infer behavior from the regex alone.
Technical reference notes
The page is scoped to JavaScript regex behavior. Compatibility review matters because unsupported syntax is a common source of deployment failures.
- MDN JavaScript regular expressions — pattern creation and use with JavaScript string methods
- MDN regex syntax cheat sheet — syntax categories for assertions, groups, and quantifiers
- Regex production review — local checklist before release
Copy result checklist
- Record the target runtime and flags before copying the pattern.
- Test positive, negative, empty, multiline, and long near-match samples.
- Inspect capture groups after every structural edit.
- Decide whether matches should be global, case-insensitive, multiline, dotall, or Unicode-aware.
- Avoid using regex as a complete parser for nested formats such as JSON, HTML, or programming languages.
- Run performance-sensitive patterns in the application environment before release.
Related guides for deeper review
Editorial update: 2026-06-01. This Regex page was expanded with flags, escaping, performance, example, and deployment-review notes so patterns can be checked against realistic positive and negative samples.