If specified, this will be used instead of the regular expression's match to search for the ident.
Supports referencing capture groups, e.g. $123 or ${name} and $ can be escaped with \\$.
Example:
- with a payment description of INV/01234567/89
- a regularExpression of (\\d*)/(\\d*) would match 01234567/89, with the capture groups 01234567 and 89
- a replaceWith of $1-$2 would yield 01234567-89
Therefor the payment would be assigned to an invoice with ident `01234567-89`.
Size must be less than or equal to 255.