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(flex.info)Deficiencies

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Deficiencies / Bugs
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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, for example, `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.