CODEWITHSUNDEEP
flex-lexer
user3108076
user3108076

Reputation: 13

Multiple matches for patterns in one token

I need to count every occurence of "ab.", "ba.", ".ab" in some text, where "." is (as usual) any character except newline.

I wrote:

%%

ab. counter++;
ba. counter++;
.ab counter++;

%%

My problem is, that for string:

abab

my counter is equal to 1 and it should be 3. It only matches "ab." (aba). What should I do?

Upvotes: 1

Views: 823

Answers (1)

SáT
SáT

Reputation: 3692

This is how flex works: it consumes the characters already matched. In this specific case, it matches ab. on aba, consumes these characters, and leaves only b in the stream.

You could theoretically try to put back characters into the stream using the yyless(n) macro, which returns all but the first n characters into the stream. In your case yyless(2) (or rather yyless(yyleng-1) ) in all rules would give you the expected results.

Alternatively, you could use the / lookahead operator: a/(b.) is matched if the current character is an "a", followed by a "b" and any character, without actually consuming the anything other than the "a". It's a powerful tool, though I was always taught that the / is dangerous, and should be avoided (with the necessary logic moved to the parser).

Flex isn't really designed for this, though. You'd be better off doing it manually in any programming language.

Upvotes: 2

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