The second EXPR is the template specification. The first time parse is called with a particular template, the template is compiled, then used to parse the source expression. Subsequent parse calls with the same template will use the previously compiled version of the template rather than compile the template again. If a variable's value is to be used as a pattern, it is enclosed in parentheses. Literal patterns are enclosed in either single or double quotes.
Patterns can contain spaces within the quotes or parentheses.
Remember that Perl starts counting position at zero, so absolute numeric patterns should be one less than in REXX to identify the same character position. Lexical variables can not be used in the template. Then pass the template to 'drop' when you're done with it. Or use the object oriented flavor discussed below. Consult your favorite REXX manual for more details on templates.