How to exclude unwanted patterns from a set of patterns?

Question

I've Just started with lex programmming. The first assignment was to seperate the vowels and consonants from a file. The rule for the vowel which I wrote was- [ aeiouAEIOU ] {return VOWEL}; For consonants, it would be tedious to write the code - [b-dB-D.....] . Is there a way like {alphabets} - {unwanted chars}?

Answer 1

Yes, there is (in flex, not in other lex implementations):

[[:alpha:]]{-}[aeiouAEIOU]

You could also use a case-insensitive pattern:

(?i:[[:alpha:]]{-}[aeiou])

The {-} operator only works with character classes. It won't work with macro definitions or multicharacter subpatterns. (These will produce a syntax error when flex tries to parse the pattern.)

For more information, see the flex manual chapter on patterns.