Regex Tester

A regular expression (regex) is a compact language for describing a shape of text rather than a literal string — "match an email address", "find any line starting with a number", "strip HTML tags" — instead of matching one fixed word. It is built into almost every programming language, most text editors, and tools like grep, so learning to read one pays off well beyond a single project. The trouble with regex is that it is notoriously easy to get subtly wrong: a pattern that looks right can fail to match an edge case, match too much, or in rare cases run so slowly on certain inputs that it locks up a page. A live tester closes that feedback loop — you see exactly what a pattern catches and misses before it goes anywhere near production code. This tool treats your test string as a JavaScript regular expression would, because that is what actually runs behind the scenes: your pattern is compiled with `new RegExp()` and executed against the text using the same engine a browser uses. The four flags map directly onto JavaScript's own flag set. "g" (global) finds every match in the string rather than stopping at the first one — this tool always searches globally internally so the match table is complete, regardless of whether you've ticked it, but leaving it off changes how capture groups behave in some downstream code, so it is still worth testing with your intended flag combination. "i" makes the match case-insensitive, so `[a-z]` also matches `A`–`Z`. "m" (multiline) changes what `^` and `$` mean: without it they anchor to the very start and end of the whole string; with it, they anchor to the start and end of each line. "s" (dotAll) makes `.` match newline characters too, which it otherwise does not — a frequent source of "why doesn't my pattern match across lines" confusion. Capture groups — the parenthesised parts of a pattern, like `(\d{4})-(\d{2})-(\d{2})` for a date — are what let you pull specific pieces out of a match rather than just confirming a match happened. The match table lists each group's captured text in order, so you can immediately check that group 1 really is the year and not, say, an empty optional group that shifted the numbering. Non-capturing groups, written `(?:...)`, are used purely for grouping alternation or repetition without adding an entry to that list — useful when you need structure but do not need the substring back. Because the whole point of this tool is to compile arbitrary user-supplied patterns, it deliberately never uses `eval()` or `new Function()` anywhere — `new RegExp()` is a data constructor, not code execution, so there is no script-injection risk from typing a pattern. The test string is HTML-escaped before matches are wrapped in a highlight tag, so pasting in text that happens to contain `<script>` or other markup cannot alter the page.