Although this may be a "feature creep" and I'm not sure whether many people find it useful or not, I would like to make a "Work in Progress" pull request, hoping this may stimulate other discussions.

As far as I know, there is no way/trick to extract a part of a string in FORM. So I experimentally implementedsubstr_(STRING,POS,LEN) macro, which is more or lesssubstr of Perl and PHP but the first character is at POS=1. (Note that it is notsubstring_(STRING,FROM,TO)).POS andLEN are somewhat extended to non-positive and negative integers, respectively. Especially,LEN=-1 means the actual length ofSTRING.

An application is defining a procedure with optional parameters:

#procedure Proc1(x,?opts)  #define expand "-1"  #do opt={`?opts'}    #ifdef `opt'      #switch `opt'        #case expand          #redefine expand "0"          #break        #case noexpand          #redefine expand "-1"          #break        #default          #if "`substr_(`opt',1,7)'" == "expand="            #redefine expand "`substr_(`opt',8,-1)'"          #else            #message Error: illegal option of Proc1: `opt'            #terminate          #endif          #break      #endswitch    #endif  #enddo  #message x=`x'  #message expand=`expand'#endprocedure

which accepts#call Proc1(x,expand=2) as well as#call Proc1(x),#call Proc1(x,expand) or#call Proc1(x,noexpand).

This branch also contains an implementation ofstrlen_(STRING), but it is not primitive in the sense that it can be implemented based onsubstr_:

#procedure StrLen(?str,dest)  #redefine `dest' "0"  #do loop=1,1    #if "`substr_(`?str',1,``dest'')'" != "`?str'"      #redefine `dest' "{``dest''+1}"      #redefine loop "0"    #endif  #enddo#endprocedure#define n ""#call StrLen(12345,n)

Other string operations likestrreplace_(STRING,OLD,NEW) can also be implemented based onsubstr_.