When I first posted the simple regex code I thought that it contained no errors. But when I tried it a few days after that with a few simple testcases (which I hadn’t considered in the test code) it failed.

ok $ match "world$" "world"
ok $ match "world$" " world"
ok $ match "world"  "hello world"

The first testcase worked. The other two didn’t, but they should have. After a bit of inspection with the trace function in Debug.Trace I found out that the function couldn’t move past the first character. I thought matchhere would take care of that, but it didn’t.

The change that is needed was in match. These lines have the error.

match reg s@(x:xs) = if matchhere reg s
                        then True
                        else matchhere reg xs

The function only tried to match at the first and second position.

match reg s@(x:xs) = if matchhere reg s
                        then True
                        else match reg xs

By changing matchhere to match, now matchhere will be called on each character of the string. Because this way the end of the string can be reached, I have to the two lines to check for the end of the string and the regex.

match [] _    = True
match _  []   = False

It seems that the code works now like it should.