8. How the input is matched

When the generated scanner is run, it analyzes its input looking for strings which match any of its patterns. If it finds more than one match, it takes the one matching the most text (for trailing context rules, this includes the length of the trailing part, even though it will then be returned to the input). If it finds two or more matches of the same length, the rule listed first in the flex input file is chosen.

Once the match is determined, the text corresponding to the match (called the token) is made available in the global character pointer yytext, and its length in the global integer yyleng. The action corresponding to the matched pattern is then executed (a more detailed description of actions follows), and then the remaining input is scanned for another match.

If no match is found, then the default rule is executed: the next character in the input is considered matched and copied to the standard output. Thus, the simplest legal flex input is:

%%

which generates a scanner that simply copies its input (one character at a time) to its output.

Note that yytext can be defined in two different ways: either as a character pointer or as a character array. You can control which definition flex uses by including one of the special directives ‘%pointer’ or ‘%array’ in the first (definitions) section of your flex input. The default is ‘%pointer’, unless you use the ‘-l’ lex compatibility option, in which case yytext will be an array. The advantage of using ‘%pointer’ is substantially faster scanning and no buffer overflow when matching very large tokens (unless you run out of dynamic memory). The disadvantage is that you are restricted in how your actions can modify yytext (see the next section), and calls to the ‘unput()’ function destroys the present contents of yytext, which can be a considerable porting headache when moving between different lex versions.

The advantage of ‘%array’ is that you can then modify yytext to your heart’s content, and calls to ‘unput()’ do not destroy yytext (see below). Furthermore, existing lex programs sometimes access yytext externally using declarations of the form:

extern char yytext[];

This definition is erroneous when used with ‘%pointer’, but correct for ‘%array’.

‘%array’ defines yytext to be an array of YYLMAX characters, which defaults to a fairly large value. You can change the size by simply #define’ing YYLMAX to a different value in the first section of your flex input. As mentioned above, with ‘%pointer’ yytext grows dynamically to accommodate large tokens. While this means your ‘%pointer’ scanner can accommodate very large tokens (such as matching entire blocks of comments), bear in mind that each time the scanner must resize yytext it also must rescan the entire token from the beginning, so matching such tokens can prove slow. yytext presently does not dynamically grow if a call to ‘unput()’ results in too much text being pushed back; instead, a run-time error results.

Also note that you cannot use ‘%array’ with C++ scanner classes (the c++ option; see below).

[ << ]

[ >> ]

[Top]

[Contents]

[Index]

[ ? ]

This document was generated on December 29, 2020 using texi2html 1.82.