How to isolate a substring in a parameter to an m4 macro

Question

Isolating the first character of a parameter in m4 (standard Unix utility) can be done with

define(`firstletter',substr(parameter, 0, 1 ))

This makes no sense unless parameter is itself defined. Indeed this example occurs, in a body for a definition, so parameter cannot be quoted. Unfortunately the parameter can contain the characters, ,' and )' but not in the first position. Whatever I try, this leads to error messages as the define sees as its firstparameter substr(C' for the case that parameter contains C,'. How can I handle this? With other words fix the above definition for the special cases I mentionned.

Answer 1

Suppose you have to isolate the first character of the the second parameter $2 in a macro main

define(`main', `the first letter is substr($2,0,1)')

main(aap, `a,b')

Thus doesnot work because of the comma.

instead define

define(`main', `
define({newdef},translit({{$2}},{,()#},{____}))dnl
the first letter is substr(newdef,0,1)')

Unless the offending character is in the first position, this kills all the troublesome characters in the remainder of the parameter. So, this is not an ideal solution.