Code:
String: ",,,,,,,,,,,,,,,,"
pass 1: ",0,,0,,0,,0,,0,,0,,0,,0," -> return to "L" marker
pass 2: ",0,0,0,0,0,0,0,0,0,0,0,0,0,0,0," -> return to "L" marker
pass 3: nothing changed, end of loop ("," at the end is changed by the last substitution to ",0")