Regex Builder
Compose regular expressions from reusable blocks and watch matches highlight live against your test text.
Regular expressions are powerful and notoriously fiddly - a misplaced backslash or a missing quantifier turns a working pattern into one that silently matches nothing. This builder removes the guesswork by assembling patterns from discrete blocks: choose a token, add literal text, attach a quantifier, and watch the pattern take shape while your test text highlights every match in real time.
Each block is either a token - digit, word character, whitespace, a character class or a boundary - or a literal string that gets escaped automatically. Every block carries an optional quantifier, from "optional" to "one or more" to "exactly three". Literals are escaped for you, so the annoying backslash bookkeeping that trips up hand-written patterns disappears.
The generated pattern updates live as you edit, and the test area re-runs against the text below, marking each match. Toggle the g, i and m flags from the pattern row. Everything runs in your browser - the pattern and your test data never leave the page, which makes the builder comfortable to use with real strings from logs, forms or APIs.
/\d\d\d/giNo matches yet - add a block or change the test text.
Features
- Assemble patterns from tokens, escaped literals and quantifiers.
- Live test text with every match highlighted as you edit.
- Automatic escaping of literal text - no backslash guessing.
- Common tokens ready to drop in: digits, word chars, whitespace, classes and boundaries.
- Quantifiers from optional (?) to one-or-more (+) to exact counts like {3}.
- Global, case-insensitive and multi-line flags as one-click toggles.
- Fully offline - patterns and test data never leave your browser.
How to Use
- 1
Add blocks
Press Token to add a predefined token, or Literal to add plain text that will be escaped for you.
- 2
Configure each block
Pick the token or type the literal, then choose a quantifier such as "one or more" to control how many times it matches.
- 3
Set flags
Enable g for all matches, i to ignore case, or m to make anchors work across multiple lines.
- 4
Test against real text
Paste your actual input into the test area and watch matches appear instantly. Tweak blocks until the pattern behaves.
Example
Usernames (word chars, one or more)
blocks: [Word char +] · flags: g, i↓
Pattern: \w+ → matches "user1", "user2", "User3", "user_42"Digits with optional dash
blocks: [Digit +][Literal "-"][Digit +] · flags: g↓
Pattern: \d+-\d+ → matches "2026-08" but not "hello"Common Problems
Forgetting to escape literals
A literal dot or plus sign means "any character" or "one or more" in regex. The builder escapes literal text automatically, so "price.total" matches exactly that string instead of behaving as a wildcard pattern.
Quantifiers applying to the wrong thing
A quantifier binds to the block it is attached to. "Digit +" means one or more digits, not "digit then optional anything". If a match is too greedy or too short, check which block owns the quantifier.
Anchoring too early
Adding a boundary token in the middle of a pattern can make it match differently than intended. Word boundaries split on non-word characters, so "\w+\b" inside a longer pattern behaves differently than at the edges.
Case sensitivity surprises
Without the i flag, a pattern built from "user" matches only lowercase. If your data mixes case, enable i rather than adding separate blocks for each spelling.
Testing only one input
A pattern that matches your sample may still be wrong for real data. Paste several representative inputs - the happy path, the empty case, and edge cases with punctuation and numbers.
Building a pattern you cannot explain
If you cannot say in a sentence what the pattern does, it will not survive maintenance. The builder keeps each block visible and separable, so the assembled expression stays understandable block by block.
Quantifiers that match too little
The * quantifier means zero or more, so "Digit *" happily matches an empty string. If a pattern silently succeeds on nothing, switch the quantifier to + (one or more) to force at least one match.
Flags carried over from a previous pattern
A pattern that worked with case-insensitive matching may rely on i without needing it. When you paste the generated pattern elsewhere, re-check which flags it ships with rather than assuming defaults.
Technical Details
Literal blocks are escaped with the standard regex escaping rules: every metacharacter - . * + ? ^ $ { } ( ) | [ ] \ - is prefixed with a backslash so it matches literally.
Tokens are inserted as raw pattern source, so a word-character token is exactly \w. Combined with a quantifier it becomes a complete atom like \w+ or [0-9]{3}.
The assembled pattern is compiled with the JavaScript RegExp constructor, and matches are collected with a global exec loop that guards against zero-width matches.
The pattern preview updates on every change and the result area re-runs automatically, giving immediate feedback while you build.
Quantifiers are applied to the token or literal they follow, and the assembled text preserves block order exactly, so reading the final pattern top to bottom reflects the blocks left to right.
Everything runs locally in the browser. Nothing is transmitted, and the builder keeps no record of the patterns or test text you use.
Frequently Asked Questions
Is this easier than writing regex by hand?
For everything except the very simplest patterns, yes - the builder handles escaping, quantifier placement and flag syntax so you can reason about blocks instead of backslashes. The generated pattern is ordinary regex you can paste anywhere.
Can I use the generated pattern in my code?
Yes. The output is a standard JavaScript regular expression. Use it directly in code or in any regex tester - it is plain pattern text, not tied to this site.
How are literal dots handled?
A dot in literal text is escaped to \., so it matches a literal dot. If you actually wanted "any character", use the Any char token instead - the builder keeps the two cases explicit.
What do the flags do?
g makes the pattern find every match instead of stopping at the first; i ignores case; m makes ^ and $ match at line boundaries within multi-line text.
Does the tool upload my test text?
No. Pattern building and testing happen entirely in your browser. The text you paste is processed locally and never transmitted, stored or logged.
What if my pattern is invalid?
The tool compiles every change and surfaces the exact error from the regex engine - for example a dangling quantifier or an unterminated class - so you can fix the offending block immediately.
Data & Privacy
Your data stays in your browser. Nothing is uploaded.
- Processing
- Local
- Upload
- None
- Server Storage
- None
- Account
- Not required
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