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).
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