Parameter start_num specifies the character at which to start the search. The first character is character number 1. If start_num is omitted, it is assumed to be 1.
Mae' r paramedr rhif_ cychwyn yn penodi' r nod i ddechrau' r chwiliad wrtho. Y nod cyntaf yw nod rhif 1. Os gadewir rhif_ cychwyn, cymerir bod yn 1.