Capturing group doesn't exist message could suggest the solution when the cause is replacement confusion

[i30817]

I was being kind of a idiot and was doing:

def replaceRoman(source, romana, number):
    source = regex.sub(rf"([\s']|^){romana}([\s,]|$)", rf"\1{number}\2", source)
    return source

This fails in both here and re because the 'number' being added changes the first capturing group

the solution is

def replaceRoman(source, romana, number):
    source = regex.sub(rf"([\s']|^){romana}([\s,]|$)", rf"\g<1>{number}\g<2>", source)
    return source

Maybe the exception message can print out the final text and the part that is supposed to be the 'failing group', then suggest the \g<#> construct? I'll be honest, i didn't immediately get 'group 13' was the error because i was automatic coding.