# E4209

Compiler diagnostic name: `duplicate_regex_match_binding`.

Duplicate regex match binding.

Each special binding in a regex match expression can be specified at most once.
Using `before` or `after` multiple times would give the same part of the match
more than one name.

## Erroneous example

```{literalinclude} /sources/error_codes/4209_error/top.mbt
:language: moonbit
```

## Suggestion

Use each binding label only once.

```{literalinclude} /sources/error_codes/4209_fixed/top.mbt
:language: moonbit
```