rule:

number [0-9]+
plus \+
minus \-
multiply \*
divide \/
lparen \(
rparen \)
whitespace [ \t\r\n]+

mapping:

number number
plus plus
minus minus
multiply multiply
divide divide
lparen lparen
rparen rparen

code:

%c%
%post_processor
    if (input->tag == slex_tag_whitespace) {
        return slex_skip;
    }
    *output = input;
    return slex_continue;
post_processor%

%c#%
%post_processor
            if (Input.Tag == SegmentTag.whitespace)
            {
                Output = null!;
                return PostProcessResult.Skip;
            }
            Output = Input;
            return PostProcessResult.Continue;
post_processor%