Regex Tester

About Regex Tester Online — Test Regular Expressions Live

This free regex tester online lets you write, test, and debug regular expressions interactively with real-time match highlighting. Type your pattern, set flags, and paste your test string — matches are highlighted instantly in the text and listed below with their exact character positions and capture group values. Whether you are learning regular expressions for the first time, debugging a complex validation pattern, or verifying that a substitution regex works correctly, this regex tester makes the process immediate and visual.

Regular expressions (regex or regexp) are a domain-specific language for describing text patterns. They appear in virtually every programming language — JavaScript, Python, Java, Ruby, Go, PHP, Perl — and in command-line tools like grep, sed, and awk. Regex patterns are used for input validation (email addresses, phone numbers, postal codes), data extraction from unstructured text, search-and-replace operations in code editors, log file parsing, URL routing in web frameworks, and text transformation pipelines. The core concepts — character classes, quantifiers, anchors, groups — are transferable across languages, even when specific syntax details differ between regex engines.

How to Use the Regex Tester

1. Enter your regex pattern in the pattern field. Do not include surrounding slashes — type just the pattern itself (e.g. \b\d{4}\b not /\b\d{4}\b/g).
2. Set any flags using the flag checkboxes or input: g (global — find all matches, not just the first), i (case-insensitive), m (multiline — ^ and $ match line starts and ends).
3. Type or paste your test string in the text area. Matches are highlighted in real time as you type in either field.
4. The match list below shows each match with its start position, end position, the matched text, and any capture group values.

How the Regex Testing Works

The tool uses JavaScript's built-in RegExp object to compile your pattern with the specified flags and execute it against your test string. Match positions and capture groups are extracted from the result and used to inject highlight spans into the rendered test string display. The tester updates on every keystroke using JavaScript event listeners — there is no form submission or server round-trip involved. All pattern execution happens in your browser tab.

Example Patterns

4-digit numbers: Pattern \b\d{4}\b with flag g — matches "1234" and "5678" in "Call 1234 or 5678 but not 123 or 12345"
Email addresses: [a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,} with flag gi
HTML tags: <[^>]+> with flag g

Tips for Writing and Testing Regular Expressions

• Always use the g (global) flag when looking for multiple matches: Without the global flag, JavaScript regex stops after the first match. If your test string has multiple instances of your pattern and you only see one match highlighted, add the g flag. This is the most common reason beginners see fewer matches than expected — the flag is required to find all occurrences.
• Use capturing groups to extract specific parts: Wrap the part of your pattern you want to extract in parentheses to create a capturing group. For example, (\d{4})-(\d{2})-(\d{2}) matches a date like "2024-03-15" and captures the year, month, and day as separate groups (group 1, 2, and 3). The match list in the regex tester shows each capture group's value separately, making it easy to verify what each group captures.
• Use \b word boundaries to avoid partial matches: The pattern \d+ matches digits anywhere — including within longer numbers. To match only standalone numbers (not parts of longer ones), wrap with word boundaries: \b\d+\b. Word boundary \b matches the position between a word character (letter, digit, underscore) and a non-word character, preventing matches inside longer strings.
• Use non-capturing groups (?:...) for grouping without extraction: If you need to group part of a pattern for quantification or alternation but do not want to capture it as a group, use (?:...) instead of (...). For example, (?:https?://) matches "http://" or "https://" as a unit for quantification without creating a capture group that appears in the match results.
• Test edge cases explicitly: A regex that works on your intended input may also match unintended strings. After writing a pattern, test it against inputs it should NOT match to verify it does not produce false positives. For email validation, test malformed addresses like "user@", "@domain.com", and "user@domain" alongside valid ones. Iteratively tighten the pattern based on the edge cases you discover.

Frequently Asked Questions about Regex Tester Online