What a regex tester shows you
A good regex tester has three main areas: the pattern input, the test string, and the results. The results show all matches highlighted within the test string, the number of matches found, and the content of any capture groups.
Real-time feedback is the key feature. As you type or edit the pattern, the matches update instantly. This lets you build patterns incrementally — start with a literal match, add quantifiers, add anchors, and see exactly what you've captured at each step.
Capture groups are shown separately so you can confirm exactly what each group extracts. Named groups ((?<year>\d{4})) make this even clearer — the group name appears alongside the matched value.
Regex flags and when to use them
The global flag (g) matches all occurrences, not just the first. Without it, match() returns only the first match. In the tester, always use g to see all matches in your test string.
The case-insensitive flag (i) makes the pattern match regardless of letter case. The pattern /hello/i matches 'hello', 'Hello', 'HELLO', and 'HeLLo'.
The multiline flag (m) changes how ^ and $ work. Without m, ^ matches only the start of the entire string. With m, ^ matches the start of each line. This matters when your test string has multiple lines.
- g — global: find all matches, not just the first
- i — case-insensitive: ignore letter case
- m — multiline: ^ and $ match line starts/ends
- s — dotAll: . matches newlines too
- u — unicode: enable full Unicode support
Building and debugging patterns step by step
Start with the simplest version of the pattern that matches the obvious cases. Don't try to write the final pattern in one go.
Add test strings for both the cases that should match and the cases that shouldn't. A good test set includes edge cases — empty strings, very long inputs, strings that are almost but not quite valid.
When a pattern matches too much or too little, use the tester to isolate which part of the pattern is responsible. Remove components one at a time until you find the problem.
The Irreva Regex Tester supports JavaScript regex syntax including named capture groups, lookaheads, lookbehinds, and Unicode property escapes. The test results update in real time as you type.