Regex Tester

Online Regex Tester with Live Match Highlighting

Regular expressions are one of the most powerful tools in a developer’s toolkit for working with text. A regex is a sequence of characters that defines a search pattern, allowing you to match, extract, and replace text with surgical precision. They are used everywhere: form validation, log file analysis, search-and-replace operations in code editors, data scraping, and input sanitization. Despite their power, regex patterns can be tricky to get right on the first try. This free online regex tester lets you write a pattern, apply it to a test string, and see every match highlighted in real time so you can iterate quickly and build correct expressions with confidence.

Whether you are validating email addresses, extracting phone numbers from a document, or writing a complex pattern to parse structured log output, being able to see matches update live as you type removes the guesswork from regex development. Instead of running your code, checking the output, adjusting the pattern, and running again, you can test and refine patterns in seconds before committing them to your codebase.

How to Use This Regex Tester

Start by entering your regular expression pattern in the Pattern field. You do not need to include delimiters or slashes; just type the pattern itself. For example, to match all words that start with a capital letter, you would enter [A-Z][a-z]+.

Next, configure the flags using the checkboxes. The Global (g) flag is enabled by default and tells the engine to find all matches in the string rather than stopping after the first one. Enable Case insensitive (i) if you want the pattern to match regardless of letter case. Turn on Multiline (m) if your test string contains multiple lines and you want the ^ and $ anchors to match the start and end of each line rather than only the start and end of the entire string.

Then paste or type your test string into the Test String field. As soon as both the pattern and test string are present, the tool evaluates the regex and highlights every match directly in the results area below. The match counter updates in real time so you can immediately see how many occurrences your pattern captures. Adjust the pattern or flags and watch the highlights change instantly, no submit button required.

Key Features

Real-time match highlighting. Matches are highlighted as you type, giving you immediate visual feedback on whether your pattern works as intended. There is no need to click a button or wait for results. This tight feedback loop makes it much faster to develop and debug complex regex patterns.

Global, case-insensitive, and multiline flags. Toggle the three most commonly used regex flags with simple checkboxes. The global flag finds all matches, the case-insensitive flag ignores letter case, and the multiline flag changes how line anchors behave. These cover the vast majority of real-world regex use cases.

Live match count. The tool displays the total number of matches found in the test string, updating with every keystroke. This is useful for verifying that your pattern is not matching too broadly or too narrowly before you use it in production code.

Clean, distraction-free interface. The tester focuses on the essentials: a pattern field, flag toggles, a test string area, and highlighted results. There are no ads, pop-ups, or unnecessary options cluttering the screen. Enter your pattern, see your matches, and get back to work.

Client-side execution. All regex evaluation happens in your browser using the native JavaScript RegExp engine. Your test strings and patterns are never sent to a server. This makes the tool safe to use with log data, user records, or any other content you would prefer to keep private.

Common Regex Use Cases

Regular expressions appear across nearly every area of software development. In form validation, patterns verify that user input matches expected formats for email addresses, phone numbers, postal codes, and dates. In search and replace, regex allows you to find and transform text patterns across an entire file or codebase, such as renaming a function or reformatting dates from one style to another. For data extraction, patterns can pull structured information out of unstructured text, like scraping prices from HTML or parsing fields from server log lines. In input sanitization, regex helps strip or escape potentially dangerous characters before they reach a database or are rendered in a browser.

Frequently Asked Questions

What are regular expressions used for?

Regular expressions are used for pattern matching in text. Developers use them to validate user input such as email addresses and phone numbers, search for patterns in files and log output, perform complex find-and-replace operations, extract structured data from unstructured text, and sanitize input by removing or escaping unwanted characters. They are supported in virtually every programming language and are built into most code editors, command-line tools like grep and sed, and database query languages.

What does the global (g) flag do in regex?

Without the global flag, a regex engine stops searching after it finds the first match in the string. With the global flag enabled, the engine continues scanning through the entire string and returns every match it finds. This is essential when you want to count occurrences, extract all instances of a pattern, or perform a replace operation on every match rather than just the first one. In this tester, the global flag is enabled by default because most use cases require finding all matches.

How do I write a regex to match email addresses?

A simple pattern like [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,} will match most standard email addresses. It looks for one or more alphanumeric characters (plus some special characters) before the @ symbol, followed by a domain name and a top-level domain of at least two letters. Keep in mind that the full email specification is extremely complex, and no single regex can validate every technically valid email address. For production use, a practical approach is to use a reasonable pattern for basic format checking and then verify the address by sending a confirmation email.

Are regular expressions the same in every programming language?

The core syntax of regular expressions is broadly similar across languages, but there are meaningful differences in the details. Most languages support common features like character classes, quantifiers, anchors, and capturing groups. However, advanced features vary. JavaScript does not support lookbehind assertions in older engines (though modern engines do), Python has a different syntax for named groups than .NET, and PCRE (used by PHP and many other tools) supports features like recursive patterns that are absent in other implementations. This tester uses the JavaScript RegExp engine, so the patterns you build here will work directly in JavaScript, Node.js, and environments that use the same engine. When porting patterns to another language, check for syntax differences in features like lookaheads, named groups, and Unicode property escapes.