string_pattern | Generate strings | Regex library

If you declare your @step variable as data type XML instead of varchar, you can use XQuery to process the data, in this case a FLOWR expression in curly braces within literal XML elements, the curly braces switch from literal mode to XPath mode:

There are no doubt many ways you could approach this. One method is simply to reconstruct the string that you want:

SAS stores character variables as fixed length strings that are padded with spaces. As a consequence string comparisons ignore trailing spaces. So x=' ' and x=' ' are the same test.

The CATS() will remove all of the leading and trailing spaces, so empty strings will generate nothing at all. It sounds like you want to treat an empty string as a single space. The TRIM() function will return a single space for an empty string. So perhaps you just want to change this:

Yes, a sequence of characters or character classes is the default, and it is of higher precedence than the OR operator |. So if you don't have anything in front or behind your sequence (clearly displayed in your cases of true|false) then you don't need them.

However, if you want to use e.g. this is true|false for "this is true" or "this is false" then the precedence will fail for "this is false" and you need to group the true|false, for instance using a non-capturing group, e.g. this is (?:true|false).

I don't see any problems with your expressions where you removed some of the parentheses. However, if you want to check them, then simply put them into an (online) checker that shows the precedence (e.g. this one) and check if the resulting "explanation" changes. The various IDE plugings for regexp testing will hopefully also provide a similar tree view for you.

Beware that you are sometimes using boundary matchers (^ and $) and sometimes you are not. I'd expect those to be either used or not used.