What's the difference between this and Extract Regex Matches?

This tool answers a yes/no question (does it match, and how many times), while Extract Regex Matches lists the actual matched text for each occurrence.

Why does match count depend on the 'g' flag?

Without the global flag, JavaScript's regex test only checks for a single match. Add 'g' to get an accurate total count of every match in the text.

What happens with an invalid pattern?

The tool reports the exact syntax error, the same message the JavaScript engine itself would throw when constructing the RegExp.

Regex Tester Online - Test If a Pattern Matches

Overview

Introduction

Before wiring a regex into code, it helps to confirm it actually matches the input you expect, and see how many times.

This tool checks both instantly.

What Is Regex Tester?

A regex tester that reports whether your pattern matches the given text, and (with the global flag) how many total matches exist.

It runs entirely client-side as part of this site's String Tools collection, so nothing you paste is ever uploaded to a server.

How Regex Tester Works

The tool constructs a RegExp from your pattern and flags, calls test() for a yes/no match result, and separately counts matches via matchAll() when the global flag is set.

The transformation happens synchronously in JavaScript the moment you type, with no network round trip involved.

When To Use Regex Tester

Use it to validate a regex pattern against sample input before using it in code, or to quickly check how many times a pattern occurs.

It's a fast way to get the answer without opening a code editor, a REPL, or writing a one-off script just to check.

Often used alongside Regex Match Extractor and Regex String Generator.

Features

Advantages

Reports the exact JavaScript regex syntax error for invalid patterns.
Shows both a yes/no match result and a total count.
Builds a fresh RegExp for the count rather than reusing the instance already tested, so a leftover lastIndex cannot skew the total.

Limitations

Match counting requires the global flag; without it, only a yes/no result is meaningful.
Reports whether and how often the pattern matched but not where, so locating a specific match needs the highlighter or extractor instead.

Examples

Testing an email pattern

Input

ada@example.com

Output

Matches

The pattern successfully matches the full input text.

Best Practices & Notes

Best Practices

Add the global flag whenever you need an accurate total match count, not just a yes/no answer.
Test against text containing the cases you expect to fail as well as those you expect to match, since a pattern that matches everything is rarely correct.
Read the reported syntax error carefully when a pattern is invalid; it names the exact position at which the parser gave up.

Developer Notes

Match counting re-runs matchAll() with a freshly constructed RegExp (rather than reusing the same instance used for test()) to avoid the shared-lastIndex statefulness that global regex objects carry between calls.

Regex Tester Use Cases

Validating a regex pattern against sample input before using it in code
Checking whether user input matches a required format
Counting how many times a pattern occurs in text

Common Mistakes

Expecting an accurate match count without adding the global flag.
Testing a pattern anchored with ^ and $ against multi-line text without the m flag, where those anchors bind to the whole string rather than to each line.

Tips

Use Extract Regex Matches afterward if you need to see the actual matched text, not just whether it matched.
A count far higher than expected usually means the pattern can match an empty string, which matches at every position in the text.

Regex Tester