Backreferences in source-highlight 2.7

I'm having trouble with backreferences in source-highlight 2.7.  If I 
create a language definition file named foo.lang:

keyword = `a(.)\1`

And then I create a test file example.foo:

aaa abb acc a** a@@

And then I run source-highlight:

source-highlight --lang-def=foo.lang example.foo

Then "aaa", "abb" and "acc" are highlighted, but "a**" and "a@@" are 
not.  Shouldn't the dot match any character?