Regex Tester – Test, Validate & Debug Online
Quickly test and debug your regular expressions with our free Regex Tester tool. Perfect for beginners and advanced developers, it lets you validate regex patterns in real time, highlight matches instantly, and understand how expressions work—making pattern building faster and easier.
Tool Purpose & Audience
The Regex Tester lets you write a regular expression pattern, paste test text, and instantly see which parts of the text match — highlighting every match in real time. It supports JavaScript regex syntax with flags including global (g), case-insensitive (i), and multiline (m) so you can test patterns exactly as they'll run in code.
Developers, data engineers, QA testers, and anyone who works with text pattern matching use this tool to build, validate, and debug regular expressions before embedding them in code — avoiding the trial-and-error cycle of running code just to test a pattern.
Real-World Use Cases
- Input validation: Build and test regex patterns for validating email addresses, phone numbers, ZIP codes, credit card numbers, and URLs before using them in form validation code.
- Log file analysis: Write patterns to extract specific data from server logs, error traces, or structured log lines — test the pattern against real log samples before running it against millions of lines.
- Data extraction and transformation: Extract specific parts of text (capture groups) — dates from unstructured text, hostnames from URLs, or variable names from code — and verify the extraction is correct before wiring it into a data pipeline.
- Find-and-replace in code editors: VS Code, IntelliJ, and other editors support regex in Find & Replace. Test the pattern and replacement string here before running a regex find-and-replace across an entire codebase.
Practical Input → Output Examples
1. Email address matching
Pattern: [\w.+-]+@[\w-]+\.[a-z]{2,} | Input: "Contact us at hello@example.com or support@site.org"
Matches: "hello@example.com", "support@site.org" (highlighted)
A common email extraction pattern — the tester shows exactly which part of each address matched, making it easy to see whether the pattern is too broad or too narrow.
2. Phone number extraction with capture groups
Pattern: `(\d3)-(\d3)-(\d4)` | Input: "Call 555-867-5309 or 800-555-0100"
Matches: "555-867-5309", "800-555-0100" with group captures shown
Capture groups let you extract parts of a match separately — here the area code, prefix, and line number are each in their own group, useful for reformatting phone numbers.
3. Case-insensitive flag (i)
Pattern: `\berror\b` with flag `i` | Input: "ERROR: disk full. Error code 28."
Matches: "ERROR", "Error" (both highlighted regardless of case)
The `i` flag makes the pattern match regardless of capitalisation — essential for log parsing where error levels appear in mixed case.
Common Mistakes & Misunderstandings
Forgetting to escape special characters: Characters like `.`, `*`, `+`, `?`, `(`, `)`, `[`, `]`, and `\` have special meaning in regex. To match a literal dot (e.g., in "file.txt"), write `\.` not `.` — a bare `.` matches any character. The most common regex bug is an unescaped `.` matching far more than intended.
The global flag (g) and lastIndex: In JavaScript, when you use the global flag and call `.exec()` or `.test()` on the same RegExp object in a loop, the regex tracks `lastIndex` between calls. Forgetting this causes the alternating match/no-match bug. When testing with the global flag, use `String.matchAll()` or reset the regex between calls.
Greedy vs lazy quantifiers: By default, `*` and `+` are greedy — they match as much as possible. `<.+>` on `<b>text</b>` matches the entire string, not just one tag. Add `?` to make it lazy: `<.+?>` matches `<b>` and `</b>` separately. Greedy matching is the second most common source of unexpected regex behavior.
Regex flavours differ across languages: This tester uses JavaScript regex syntax. Python, Java, PHP, and other languages have slightly different syntax for look-aheads, named groups, and certain character classes. A regex that works here may need small adjustments for a different language runtime.
How does our Regex Tester Tool work?
Our Regex Tester simplifies working with regular expressions. Enter your regex pattern and test text, and the tool highlights matches instantly. It’s great for debugging complex patterns or learning the basics. How to use: 1) Enter your regex in the pattern field. 2) Type or paste text to test. 3) The tool automatically highlights matches and shows groups. 4) Use flags (g, i, m, s, u, y) to modify behavior. Invalid patterns show instant feedback so you can learn and iterate quickly.
What are the rules of regular expressions?
Regex basics include: 1) Anchors ^ and $ match start/end of line. 2) Quantifiers *, +, ?, and {n} control repetition. 3) Character classes like [a-z] and shortcuts like \d match sets and digits. 4) Grouping and capturing with ( ). 5) Alternation using |. 6) Escaping special characters with \ (e.g., \.).
What is a regular expression?
A regular expression (regex) is a special text pattern used to describe search criteria. Think of it like advanced wildcards that help you find or transform text. Example: /.*\.txt$/ matches all lines ending with .txt.
Which type of regular expression does this tool work with?
This tool uses JavaScript regular expressions and supports standard JS syntax and flags. You can use it to test patterns for JavaScript, and often adapt them for PHP, Python, Ruby, Java, and other languages with similar regex engines.
What can I use a regular expression for?
Use regex to: validate input (e.g., emails, phone numbers), search for patterns in text (e.g., find all URLs), extract data from strings, replace or reformat text, and split text into arrays based on patterns.
Can I use this tool for learning regex?
Absolutely. The tool is designed for both beginners and advanced users. Try different patterns, toggle flags, and see results instantly to build your understanding of regex.
Does this tool support regex flags?
Yes. Supported flags include: g (global), i (case-insensitive), m (multiline), s (dotAll), u (Unicode), and y (sticky).
Is my data safe?
Yes. All processing is done in your browser. No data is sent to any server.