(flex.info)Debugging Options

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16.5 Debugging Options
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'-b, --backup, '%option backup''
     Generate backing-up information to 'lex.backup'.  This is a list of
     scanner states which require backing up and the input characters on
     which they do so.  By adding rules one can remove backing-up
     states.  If _all_ backing-up states are eliminated and '-Cf' or
     '-CF' is used, the generated scanner will run faster (see the
     '--perf-report' flag).  Only users who wish to squeeze every last
     cycle out of their scanners need worry about this option.  (Note:
     Performance).

'-d, --debug, '%option debug''
     makes the generated scanner run in "debug" mode.  Whenever a
     pattern is recognized and the global variable 'yy_flex_debug' is
     non-zero (which is the default), the scanner will write to 'stderr'
     a line of the form:

              -accepting rule at line 53 ("the matched text")

     The line number refers to the location of the rule in the file
     defining the scanner (i.e., the file that was fed to flex).
     Messages are also generated when the scanner backs up, accepts the
     default rule, reaches the end of its input buffer (or encounters a
     NUL; at this point, the two look the same as far as the scanner's
     concerned), or reaches an end-of-file.

'-p, --perf-report, '%option perf-report''
     generates a performance report to 'stderr'.  The report consists of
     comments regarding features of the 'flex' input file which will
     cause a serious loss of performance in the resulting scanner.  If
     you give the flag twice, you will also get comments regarding
     features that lead to minor performance losses.

     Note that the use of 'REJECT', and variable trailing context (Note:
     Limitations) entails a substantial performance penalty; use of
     'yymore()', the '^' operator, and the '--interactive' flag entail
     minor performance penalties.

'-s, --nodefault, '%option nodefault''
     causes the _default rule_ (that unmatched scanner input is echoed
     to 'stdout)' to be suppressed.  If the scanner encounters input
     that does not match any of its rules, it aborts with an error.
     This option is useful for finding holes in a scanner's rule set.

'-T, --trace, '%option trace''
     makes 'flex' run in "trace" mode.  It will generate a lot of
     messages to 'stderr' concerning the form of the input and the
     resultant non-deterministic and deterministic finite automata.
     This option is mostly for use in maintaining 'flex'.

'-w, --nowarn, '%option nowarn''
     suppresses warning messages.

'-v, --verbose, '%option verbose''
     specifies that 'flex' should write to 'stderr' a summary of
     statistics regarding the scanner it generates.  Most of the
     statistics are meaningless to the casual 'flex' user, but the first
     line identifies the version of 'flex' (same as reported by
     '--version'), and the next line the flags used when generating the
     scanner, including those that are on by default.

'--warn, '%option warn''
     warn about certain things.  In particular, if the default rule can
     be matched but no default rule has been given, the flex will warn
     you.  We recommend using this option always.