(flex.info)Matching

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7 How the Input Is Matched
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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 (Note: Actions),
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 valid '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' (Note: Actions), 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'
(Note: Actions).  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'.

   The '%array' declaration 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
(Note: Cxx).