...

set trimmedString = stringSet.s1.replace("^[ ]+", "");

set trimmedString = stringSet.s1.replace("[ ]+$", "");

set trimmedString = stringSet.s1.replace("^[ ]+|[ ]+$", "");

set trimmedString = stringSet.s1.replace("^[ \t]+", "");

Replacing Parts of a Regular Expression

If you want to replace parts of a string that contains a regular expression, you may run into the problem that the Runtime will interpret the matching part.
In this case, adorn the matching part expression with \Q and \E to mark it as a literal string, so the Runtime will simply take it as it is for comparison (see also Meta Characters).

set pdfFilenamePattern = "invoice_\d{8}\d{6}\d{5}((\.pdf)|(\.PDF))";
set matchingPart       = "\d{8}\d{6}\d{5}";
set replacement        = "12345678_123456_12345";

set changed = replace(pdfFilenamePattern, concat('\Q', matchingPart, '\E'), replacement);

The same applies if the replacement contains a regular expression.