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 24 Limitations
 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      |
    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.
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