Regex Tester - How to Write and Test Regular Expressions Online

Regular expressions (regex) are powerful but notoriously hard to read and debug. One misplaced character can completely change the match behavior. A good regex tester lets you see matches in real time - essential for writing correct patterns.

Why Use a Regex Tester?

• Instant feedback - see matches highlighted as you type the pattern
• Debugging - identify why a pattern is not matching
• Learning - experiment with patterns and see what they do
• Group capture - inspect capture groups and their values
• Validation - test patterns before deploying to production code

Common Regex Patterns

• Email: /^[^\s@]+@[^\s@]+\.[^\s@]+$/
• URL: /^https?:\/\/(www\.)?[\w-]+\.[\w.-]+[\/\w.-]*$/
• Phone (US): /^\(\d{3}\)\s?\d{3}-\d{4}$/
• IP address: /^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/
• Date (YYYY-MM-DD): /^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$/
• Hex color: /^#?([0-9A-Fa-f]{3}|[0-9A-Fa-f]{6})$/

Regex Flags Explained

• g (global) - find all matches, not just the first
• i (case-insensitive) - match regardless of case
• m (multiline) - ^ and $ match line boundaries, not just string boundaries
• s (dotAll) - . also matches newlines
• u (unicode) - enable Unicode property escapes

Free Online Regex Tester

Use SnapSum Regex Tester - a browser-based tool that highlights matches in real time, shows capture groups, and requires no account.

• Live match highlighting as you type
• Capture group inspection
• Toggle flags: g, i, m, s, u
• Match count and position info
• Privacy-first: no data leaves your browser

Step-by-Step: Test a Regex

1. Open Regex Tester.
2. Enter your regex pattern in the pattern field.
3. Select flags (g, i, m, etc.).
4. Paste your test string in the input area.
5. See highlighted matches and capture groups instantly.

Regex Cheat Sheet

• . - any character (except newline, unless s flag)
• \d - digit [0-9]
• \w - word character [a-zA-Z0-9_]
• \s - whitespace
• ^ - start of string/line
• $ - end of string/line
• * - 0 or more
• + - 1 or more
• ? - 0 or 1
• {n,m} - between n and m
• (...) - capture group
• (?:...) - non-capturing group
• [abc] - character class
• | - alternation (OR)

Common Mistakes

• Forgetting to escape - . matches any character; use \. to match a literal dot
• Greedy by default - .* matches as much as possible. Use .*? for lazy matching.
• ReDoS - nested quantifiers like (a+)+ can cause catastrophic backtracking on certain inputs. Always test with edge cases.