Regular Expression Has Syntax Error in Javascript Only

What am I doing wrong?

The regular expression you are creating is

^/courses(/[a-z0-9-]+){2,3}(/|.jsp)?(?.*)?$
                                    ^^^^^

Note the (? part, which is illegal (and hence the error).

\ is the escape character in string literals and regular expression. The parser doesn't know that there is a regular expression in the string literal, the \ is evaluated as part of the string literal, so \? becomes ?. Simple test:

> "\?"
"?"

You have the same problem with \.jsp, which becomes .jsp. The . is not illegal at that position, but it will match every character instead of a ..

If you want to pass it along to the regular expression, you have to escape it:

"^/courses(/[a-z0-9\\-]+){2,3}(/|\\.jsp)?(\\?.*)?$"
                                 ^        ^

Or you simple you use an expression literal (which requires you do escape the / though):

.replace(/^\/courses(\/[a-z0-9-]+){2,3}(\/|\.jsp)?(\?.*)?$/, "abc");