CL-INTERPOL and CL-PPCRE

Hi I would like to munge regex strings to interpolate substrings in them (I know you can do that in CL-PPCRE using the SEXP based regexps). Something equivalent *(defvar L "[A-Za-z]")* *(scan #?"|${L}+|" " |foobar|...")* But I would like to build the interpolated string dynamically, i.e., *before* I create the *L* variable. In other words, I would like to interpolate at the latest possible time. I know I am doing something wrong. Thanks -- Marco