Could I specify pattern match priority in lex code? - match

Question

I've got a related thread in the site(My lex pattern doesn't work to match my input file, how to correct it?)
The problems I met, is about how "greedy" lex will do pattern match, e.g. I've got my lex file:
$ cat b.l
%{
#include<stdio.h>
%}
%%
"12" {printf("head\n");}
"34" {printf("tail\n");}
.* {printf("content\n");}
%%
What I wish to say is, when meet "12", print "head"; when meet "34", print "tail", otherwise print "content" for the longest match that doesn't contain either "12" or "34".
But the fact was, ".*" was a greedy match that whatever I input, it prints "content".
My requirement is, when I use
12sdf2dfsd3sd34
as input, the output should be
head
content
tail
So seems there're 2 possible ways:
1, To specify a match priority for ".*", it should work only when neither "12" and "34" works to match. Does lex support "priority"?
2, to change the 3rd expression, as to match any contiguous string that doesn't contain sub-string of "12", or "34". But how to write this regular expression?

Answer 1

Does (f)lex support priority?
(F)lex always produces the longest possible match. If more than one rule matches the same longest match, the first one is chosen, so in that case it supports priority. But it does not support priority for shorter matches, nor does it implement non-greedy matching.
How to match a string which does not contain one or more sequences?
You can, with some work, create a regular expression which matches a string not containing specified substrings, but it is not particularly easy and (f)lex does not provide a simple syntax for such regular expressions.
A simpler (but slightly less efficient) solution is to match the string in pieces. As a rough outline, you could do the following:
"12" { return HEAD; }
"34" { if (yyleng > 2) {
yyless(yyleng - 2);
return CONTENT;
}
else
return TAIL;
}
.|\n { yymore(); }
This could be made more efficient by matching multiple characters when there is not chance of skipping a delimiter; change the last rule to:
.|[^13]+ { yymore(); }
yymore() causes the current token to be retained, so that the next match appends to the current token rather than starting a new token. yyless(x) returns all but the first x characters to the input stream; in this case, that is used to cause the end delimiter 34 to be rescanned after the CONTENT token is identified.
(That assumes you actually want to tokenize the input stream, rather than just print a debugging message, which is why I called it an outline solution.)