The following lines take an inordinate amount of time to process with the FortranLexer:

foobar = 'foo'//'\'//'bar'
!\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

See demo. The time increases with each added slash in the comment line.

As discussed in #1508, this is likely a catastrophic backtracking problem. The problem is linked to the issue with backslashes in string constants discussed in #1508, since replacing '\' with '\\' in the first line appears to result in a normal run time.