# GB.SubstString

`char * GB.SubstString ( const char * pattern , long len , GB_SUBST_CALLBACK callback )`

Substitutes elements in a pattern and returns the resulting string.

- pattern points at the pattern.
- len is the pattern length.
- callback is a function that will be called to do each substitution.

The substituted elements are the `&` character followed by an index between `1` and `9`. This index is passed to the callback.

See GB_SUBST_CALLBACK for more information.