A regex tester makes a pattern easier to inspect by showing matches and capture groups as you work. Start with a few real examples, but also add near misses—the strings that should not match.
Check the selected regex engine and flags. JavaScript, PCRE, and other engines differ in supported features; case sensitivity, multiline anchors, and global matching can change results. Test Unicode, line endings, empty strings, and optional groups where they matter. Anchors and word boundaries are particularly easy to misread when a sample spans multiple lines.
A highlighted match proves only that the pattern matched that sample under those settings. Before using it for validation or replacement, run a small explicit test set of expected matches and expected rejections in the same engine that production uses.
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