My pattern matches in another tool but not here

different regex engines have different syntax. Python's `re` module, PCRE (used by PHP/Perl), and JavaScript's `RegExp` are not identical. Named groups in Python use `(?P ...)` while JavaScript uses `(? ...)`. Check engine-specific syntax.

Matches highlight but the capture group table is empty

your pattern has no capture groups. Parentheses create groups: `(\d+)` captures digits. If you're using non-capturing groups `(?:...)`, they won't appear in the table.

The pattern shows an error but it looks correct

JavaScript regex syntax has specific rules. Common issues: unescaped `{` or `}` (needs `\{`), invalid quantifier like `{,3}` (needs `{0,3}`), or unsupported syntax for your browser.

I can't match across lines

by default, `.` doesn't match `\n`. Enable the `s` flag. Or use `[\s\S]` as a portable "match anything including newlines" pattern that works everywhere.

My pattern matches too much

you're probably using greedy quantifiers. Switch `*` to `*?`, `+` to `+?`, or use more specific character classes instead of `.`.

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