How to Test Regular Expressions Online

Regular expressions are one of the most powerful tools in programming — and one of the most frustrating to get right. A regex tester lets you build and debug patterns interactively instead of running your code, checking the output, and guessing what went wrong.

Why use a regex tester

Writing regex in your code editor means you only see errors at runtime. A tester shows you:

• Live match highlighting — see exactly which parts of your text match as you type the pattern
• Capture groups — see what each group captures without writing debug output
• Match details — exact positions, lengths, and content of every match
• Replacement preview — see the result of find-and-replace before committing to it

How to test regex online

1. Enter your pattern — type the regex in the pattern field. Toggle flags (g for global, i for case-insensitive, m for multiline) as needed.
2. Paste your test text — enter the text you want to match against. Matches highlight in real time.
3. View results — see all matches with capture groups listed below. Use the "Replace with" field to test replacements.

Common regex patterns worth knowing

Email address (basic):

[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}

URL:

https?://[^\s]+

Phone number (US):

\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}

Date (YYYY-MM-DD):

\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01])

IP address (IPv4):

\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b

Tips for writing better regex

• Start simple — get a basic pattern working first, then add complexity. Trying to write the perfect regex in one shot rarely works.
• Use the global flag (g) — without it, the tester stops at the first match. With g, you see all matches in the text.
• Test edge cases — your regex might match the obvious cases but fail on empty strings, special characters, or boundary conditions. Add these to your test text.
• Escape special characters — characters like ., *, +, ?, (, ), [, ], {, }, \, ^, $, and | have special meaning in regex. To match them literally, prefix with a backslash.
• Use non-capturing groups — if you need parentheses for grouping but do not need the capture, use (?:...) instead of (...). This keeps your match results cleaner.