(flex.info)Limitations

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24 Limitations
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Some trailing context patterns cannot be properly matched and generate
warning messages ('dangerous trailing context').  These are patterns
where the ending of the first part of the rule matches the beginning of
the second part, such as 'zx*/xy*', where the 'x*' matches the 'x' at
the beginning of the trailing context.  (Note that the POSIX draft
states that the text matched by such patterns is undefined.)  For some
trailing context rules, parts which are actually fixed-length are not
recognized as such, leading to the abovementioned performance loss.  In
particular, parts using '|' or '{n}' (such as 'foo{3}') are always
considered variable-length.  Combining trailing context with the special
'|' action can result in _fixed_ trailing context being turned into the
more expensive _variable_ trailing context.  For example, in the
following:

         %%
         abc      |
         xyz/def

   Use of 'unput()' invalidates yytext and yyleng, unless the '%array'
directive or the '-l' option has been used.  Pattern-matching of 'NUL's
is substantially slower than matching other characters.  Dynamic
resizing of the input buffer is slow, as it entails rescanning all the
text matched so far by the current (generally huge) token.  Due to both
buffering of input and read-ahead, you cannot intermix calls to
'<stdio.h>' routines, such as, getchar(), with 'flex' rules and expect
it to work.  Call 'input()' instead.  The total table entries listed by
the '-v' flag excludes the number of table entries needed to determine
what rule has been matched.  The number of entries is equal to the
number of DFA states if the scanner does not use 'REJECT', and somewhat
greater than the number of states if it does.  'REJECT' cannot be used
with the '-f' or '-F' options.

   The 'flex' internal algorithms need documentation.