# Regex tester

URL: https://shahid09.is-a.dev/tools/regex-tester/
Author: Muhammad Shahid (https://shahid09.is-a.dev/)
Free tool · runs in your browser

Test a regular expression against your text and see every match, its position and captured groups, with an optional replacement.

**Quick answer:** Type a regular expression, choose flags and paste your text. The tester lists every match with its position and captured groups, and shows the result of a replacement where $1 means group 1. It uses your browser's JavaScript engine, so what works here works in JavaScript and TypeScript; most simple patterns also work in Python, Java and PHP.

## How to use it

1. **Type the pattern.** Without the surrounding slashes, for example \d{3}-\d{4}.
2. **Pick flags and paste text.** g finds all matches, i ignores case, m makes ^ and $ match each line.
3. **Read the matches.** Each match shows its position and groups; add a replacement to see the changed text.

## Example

Finding email addresses and capturing the name and domain:

Input:

```
Pattern: (\w+)@(\w+)\.com   Flags: g
Text:    mail sara@example.com or ali@test.com
```

Output:

```
2 matches
1. sara@example.com  @5   $1=sara  $2=example
2. ali@test.com      @25  $1=ali   $2=test
```

## Common errors and how to fix them

- **Invalid regular expression: Unterminated group**: A ( has no matching ). Count the brackets, or escape a literal bracket as \(.
- **Invalid regular expression: Nothing to repeat**: A quantifier such as *, + or ? has nothing before it, as in *abc. Escape it as \* to match the character itself.
- **Only the first match is found**: The g flag is off. Tick g to find every match.
- **The page freezes**: Nested quantifiers such as (a+)+$ cause catastrophic backtracking on text that almost matches. Remove the nesting or make the pattern more specific.

## Frequently asked questions

### What do the flags g, i, m, s and u mean?

g finds all matches instead of the first; i ignores case; m makes ^ and $ match at line starts and ends; s lets . match newlines; u enables full Unicode matching.

### Why does my regex only find one match?

Without the g flag, JavaScript stops after the first match. Tick g.

### How do I match an email address?

A practical pattern is [\w.+-]+@[\w-]+\.[\w.]+ . Fully validating every possible email with regex is not practical; send a confirmation email instead.

### How do I use a captured group in a replacement?

Wrap part of the pattern in parentheses and use $1, $2 and so on in the replacement. Named groups (?<year>\d{4}) are used as $<year>.

### How do I match a literal dot or bracket?

Escape it with a backslash: \. matches a dot, \( a bracket. Without the backslash, . matches any character.

### What is the difference between .* and .*?

.* is greedy and matches as much as possible; .*? is lazy and matches as little as possible. Use the lazy form to stop at the first closing tag or quote.

### What is a lookahead?

A check that does not consume text: \d+(?= USD) matches a number only if " USD" follows. (?! ) is the negative version.

### Does this work for Python regex?

Mostly. The basic syntax is the same; differences include named groups, written (?P<name>) in older Python, and some flags.

### Why does the page freeze with some patterns?

Nested quantifiers such as (a+)+ can take exponential time on text that almost matches. This is called catastrophic backtracking; simplify the pattern.

### Is my text uploaded?

No. The regex runs in your browser.

## Related

- [Case converter](https://shahid09.is-a.dev/tools/case-converter/)
- [JSON formatter and validator](https://shahid09.is-a.dev/tools/json-formatter/)
- [URL encoder and decoder](https://shahid09.is-a.dev/tools/url-encoder-decoder/)