Regex Tester

How it works

Enter a pattern and test string. The tool compiles your regex with the selected flags and runs it in a Web Worker guarded by a 300ms timeout. Matches are mapped back to the input for underline-and-mark highlighting, and replace mode applies your replacement string to produce a preview output.

Benefits

• Runs locally with no upload step
• Worker timeout reduces catastrophic backtracking risk
• Accessible highlights use underline plus background
• Live match count announced via aria-live region
• Match and replace modes with standard JS flags

Use cases

• Validate email, URL, or log-line patterns before shipping code
• Debug capture groups and replacement backreferences
• Compare global versus single-match behavior quickly
• Teach regex with instant visual feedback

Best practices

• Start with a simple pattern and add complexity incrementally
• Enable the u flag when matching Unicode text
• Use non-greedy quantifiers to avoid runaway backtracking
• Test edge cases like empty strings and repeated delimiters

Common mistakes

• Forgetting the g flag when expecting all matches
• Leaving special characters unescaped in literal searches
• Using greedy .* where a bounded character class is safer
• Assuming multiline mode without enabling the m flag

Privacy

All regex compilation and execution happen in your browser inside a dedicated worker when supported. ToolHub does not receive your patterns or test strings. Optional LocalStorage history stores inputs only if you explicitly enable and save them on this device.