782 lines
20 KiB
Groff
782 lines
20 KiB
Groff
.TH FLEX 1 "26 May 1990" "Version 2.3"
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.SH NAME
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flex - fast lexical analyzer generator
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.SH SYNOPSIS
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.B flex
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.B [-bcdfinpstvFILT8 -C[efmF] -Sskeleton]
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.I [filename ...]
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.SH DESCRIPTION
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.I flex
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is a tool for generating
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.I scanners:
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programs which recognized lexical patterns in text.
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.I flex
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reads
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the given input files, or its standard input if no file names are given,
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for a description of a scanner to generate. The description is in
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the form of pairs
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of regular expressions and C code, called
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.I rules. flex
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generates as output a C source file,
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.B lex.yy.c,
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which defines a routine
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.B yylex().
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This file is compiled and linked with the
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.B -lfl
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library to produce an executable. When the executable is run,
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it analyzes its input for occurrences
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of the regular expressions. Whenever it finds one, it executes
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the corresponding C code.
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.LP
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For full documentation, see
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.B flexdoc(1).
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This manual entry is intended for use as a quick reference.
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.SH OPTIONS
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.I flex
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has the following options:
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.TP
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.B -b
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Generate backtracking information to
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.I lex.backtrack.
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This is a list of scanner states which require backtracking
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and the input characters on which they do so. By adding rules one
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can remove backtracking states. If all backtracking states
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are eliminated and
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.B -f
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or
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.B -F
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is used, the generated scanner will run faster.
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.TP
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.B -c
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is a do-nothing, deprecated option included for POSIX compliance.
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.IP
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.B NOTE:
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in previous releases of
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.I flex
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.B -c
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specified table-compression options. This functionality is
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now given by the
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.B -C
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flag. To ease the the impact of this change, when
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.I flex
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encounters
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.B -c,
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it currently issues a warning message and assumes that
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.B -C
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was desired instead. In the future this "promotion" of
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.B -c
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to
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.B -C
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will go away in the name of full POSIX compliance (unless
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the POSIX meaning is removed first).
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.TP
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.B -d
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makes the generated scanner run in
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.I debug
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mode. Whenever a pattern is recognized and the global
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.B yy_flex_debug
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is non-zero (which is the default), the scanner will
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write to
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.I stderr
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a line of the form:
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.nf
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--accepting rule at line 53 ("the matched text")
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.fi
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The line number refers to the location of the rule in the file
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defining the scanner (i.e., the file that was fed to flex). Messages
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are also generated when the scanner backtracks, accepts the
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default rule, reaches the end of its input buffer (or encounters
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a NUL; the two look the same as far as the scanner's concerned),
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or reaches an end-of-file.
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.TP
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.B -f
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specifies (take your pick)
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.I full table
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or
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.I fast scanner.
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No table compression is done. The result is large but fast.
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This option is equivalent to
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.B -Cf
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(see below).
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.TP
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.B -i
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instructs
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.I flex
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to generate a
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.I case-insensitive
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scanner. The case of letters given in the
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.I flex
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input patterns will
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be ignored, and tokens in the input will be matched regardless of case. The
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matched text given in
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.I yytext
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will have the preserved case (i.e., it will not be folded).
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.TP
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.B -n
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is another do-nothing, deprecated option included only for
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POSIX compliance.
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.TP
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.B -p
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generates a performance report to stderr. The report
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consists of comments regarding features of the
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.I flex
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input file which will cause a loss of performance in the resulting scanner.
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.TP
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.B -s
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causes the
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.I default rule
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(that unmatched scanner input is echoed to
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.I stdout)
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to be suppressed. If the scanner encounters input that does not
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match any of its rules, it aborts with an error.
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.TP
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.B -t
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instructs
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.I flex
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to write the scanner it generates to standard output instead
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of
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.B lex.yy.c.
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.TP
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.B -v
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specifies that
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.I flex
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should write to
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.I stderr
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a summary of statistics regarding the scanner it generates.
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.TP
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.B -F
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specifies that the
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.ul
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fast
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scanner table representation should be used. This representation is
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about as fast as the full table representation
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.ul
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(-f),
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and for some sets of patterns will be considerably smaller (and for
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others, larger). See
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.B flexdoc(1)
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for details.
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.IP
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This option is equivalent to
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.B -CF
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(see below).
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.TP
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.B -I
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instructs
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.I flex
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to generate an
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.I interactive
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scanner, that is, a scanner which stops immediately rather than
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looking ahead if it knows
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that the currently scanned text cannot be part of a longer rule's match.
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Again, see
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.B flexdoc(1)
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for details.
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.IP
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Note,
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.B -I
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cannot be used in conjunction with
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.I full
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or
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.I fast tables,
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i.e., the
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.B -f, -F, -Cf,
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or
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.B -CF
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flags.
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.TP
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.B -L
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instructs
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.I flex
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not to generate
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.B #line
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directives in
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.B lex.yy.c.
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The default is to generate such directives so error
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messages in the actions will be correctly
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located with respect to the original
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.I flex
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input file, and not to
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the fairly meaningless line numbers of
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.B lex.yy.c.
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.TP
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.B -T
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makes
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.I flex
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run in
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.I trace
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mode. It will generate a lot of messages to
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.I stdout
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concerning
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the form of the input and the resultant non-deterministic and deterministic
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finite automata. This option is mostly for use in maintaining
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.I flex.
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.TP
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.B -8
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instructs
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.I flex
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to generate an 8-bit scanner.
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On some sites, this is the default. On others, the default
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is 7-bit characters. To see which is the case, check the verbose
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.B (-v)
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output for "equivalence classes created". If the denominator of
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the number shown is 128, then by default
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.I flex
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is generating 7-bit characters. If it is 256, then the default is
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8-bit characters.
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.TP
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.B -C[efmF]
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controls the degree of table compression.
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.IP
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.B -Ce
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directs
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.I flex
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to construct
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.I equivalence classes,
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i.e., sets of characters
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which have identical lexical properties.
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Equivalence classes usually give
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dramatic reductions in the final table/object file sizes (typically
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a factor of 2-5) and are pretty cheap performance-wise (one array
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look-up per character scanned).
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.IP
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.B -Cf
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specifies that the
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.I full
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scanner tables should be generated -
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.I flex
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should not compress the
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tables by taking advantages of similar transition functions for
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different states.
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.IP
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.B -CF
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specifies that the alternate fast scanner representation (described in
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.B flexdoc(1))
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should be used.
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.IP
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.B -Cm
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directs
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.I flex
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to construct
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.I meta-equivalence classes,
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which are sets of equivalence classes (or characters, if equivalence
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classes are not being used) that are commonly used together. Meta-equivalence
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classes are often a big win when using compressed tables, but they
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have a moderate performance impact (one or two "if" tests and one
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array look-up per character scanned).
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.IP
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A lone
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.B -C
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specifies that the scanner tables should be compressed but neither
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equivalence classes nor meta-equivalence classes should be used.
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.IP
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The options
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.B -Cf
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or
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.B -CF
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and
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.B -Cm
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do not make sense together - there is no opportunity for meta-equivalence
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classes if the table is not being compressed. Otherwise the options
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may be freely mixed.
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.IP
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The default setting is
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.B -Cem,
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which specifies that
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.I flex
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should generate equivalence classes
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and meta-equivalence classes. This setting provides the highest
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degree of table compression. You can trade off
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faster-executing scanners at the cost of larger tables with
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the following generally being true:
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.nf
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slowest & smallest
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-Cem
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-Cm
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-Ce
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-C
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-C{f,F}e
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-C{f,F}
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fastest & largest
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.fi
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.IP
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.B -C
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options are not cumulative; whenever the flag is encountered, the
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previous -C settings are forgotten.
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.TP
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.B -Sskeleton_file
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overrides the default skeleton file from which
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.I flex
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constructs its scanners. You'll never need this option unless you are doing
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.I flex
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maintenance or development.
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.SH SUMMARY OF FLEX REGULAR EXPRESSIONS
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The patterns in the input are written using an extended set of regular
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expressions. These are:
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.nf
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x match the character 'x'
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. any character except newline
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[xyz] a "character class"; in this case, the pattern
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matches either an 'x', a 'y', or a 'z'
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[abj-oZ] a "character class" with a range in it; matches
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an 'a', a 'b', any letter from 'j' through 'o',
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or a 'Z'
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[^A-Z] a "negated character class", i.e., any character
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but those in the class. In this case, any
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character EXCEPT an uppercase letter.
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[^A-Z\\n] any character EXCEPT an uppercase letter or
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a newline
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r* zero or more r's, where r is any regular expression
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r+ one or more r's
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r? zero or one r's (that is, "an optional r")
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r{2,5} anywhere from two to five r's
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r{2,} two or more r's
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r{4} exactly 4 r's
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{name} the expansion of the "name" definition
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(see above)
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"[xyz]\\"foo"
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the literal string: [xyz]"foo
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\\X if X is an 'a', 'b', 'f', 'n', 'r', 't', or 'v',
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then the ANSI-C interpretation of \\x.
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Otherwise, a literal 'X' (used to escape
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operators such as '*')
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\\123 the character with octal value 123
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\\x2a the character with hexadecimal value 2a
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(r) match an r; parentheses are used to override
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precedence (see below)
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rs the regular expression r followed by the
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regular expression s; called "concatenation"
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r|s either an r or an s
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r/s an r but only if it is followed by an s. The
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s is not part of the matched text. This type
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of pattern is called as "trailing context".
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^r an r, but only at the beginning of a line
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r$ an r, but only at the end of a line. Equivalent
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to "r/\\n".
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<s>r an r, but only in start condition s (see
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below for discussion of start conditions)
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<s1,s2,s3>r
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same, but in any of start conditions s1,
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s2, or s3
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<<EOF>> an end-of-file
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<s1,s2><<EOF>>
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an end-of-file when in start condition s1 or s2
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.fi
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The regular expressions listed above are grouped according to
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precedence, from highest precedence at the top to lowest at the bottom.
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Those grouped together have equal precedence.
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.LP
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Some notes on patterns:
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.IP -
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Negated character classes
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.I match newlines
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unless "\\n" (or an equivalent escape sequence) is one of the
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characters explicitly present in the negated character class
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(e.g., "[^A-Z\\n]").
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.IP -
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A rule can have at most one instance of trailing context (the '/' operator
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or the '$' operator). The start condition, '^', and "<<EOF>>" patterns
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can only occur at the beginning of a pattern, and, as well as with '/' and '$',
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cannot be grouped inside parentheses. The following are all illegal:
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.nf
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foo/bar$
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foo|(bar$)
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foo|^bar
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<sc1>foo<sc2>bar
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.fi
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.SH SUMMARY OF SPECIAL ACTIONS
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In addition to arbitrary C code, the following can appear in actions:
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.IP -
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.B ECHO
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copies yytext to the scanner's output.
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.IP -
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.B BEGIN
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followed by the name of a start condition places the scanner in the
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corresponding start condition.
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.IP -
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.B REJECT
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directs the scanner to proceed on to the "second best" rule which matched the
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input (or a prefix of the input).
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.B yytext
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and
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.B yyleng
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are set up appropriately. Note that
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.B REJECT
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is a particularly expensive feature in terms scanner performance;
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if it is used in
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.I any
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of the scanner's actions it will slow down
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.I all
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of the scanner's matching. Furthermore,
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.B REJECT
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cannot be used with the
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.I -f
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or
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.I -F
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options.
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.IP
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Note also that unlike the other special actions,
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.B REJECT
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is a
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.I branch;
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code immediately following it in the action will
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.I not
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be executed.
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.IP -
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.B yymore()
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tells the scanner that the next time it matches a rule, the corresponding
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token should be
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.I appended
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onto the current value of
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.B yytext
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rather than replacing it.
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.IP -
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.B yyless(n)
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returns all but the first
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.I n
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characters of the current token back to the input stream, where they
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will be rescanned when the scanner looks for the next match.
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.B yytext
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and
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.B yyleng
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are adjusted appropriately (e.g.,
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.B yyleng
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will now be equal to
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.I n
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).
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.IP -
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.B unput(c)
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puts the character
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.I c
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back onto the input stream. It will be the next character scanned.
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.IP -
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.B input()
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reads the next character from the input stream (this routine is called
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.B yyinput()
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if the scanner is compiled using
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.B C++).
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.IP -
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.B yyterminate()
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can be used in lieu of a return statement in an action. It terminates
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the scanner and returns a 0 to the scanner's caller, indicating "all done".
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.IP
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By default,
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.B yyterminate()
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is also called when an end-of-file is encountered. It is a macro and
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may be redefined.
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.IP -
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.B YY_NEW_FILE
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is an action available only in <<EOF>> rules. It means "Okay, I've
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set up a new input file, continue scanning".
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.IP -
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.B yy_create_buffer( file, size )
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takes a
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.I FILE
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pointer and an integer
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.I size.
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It returns a YY_BUFFER_STATE
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handle to a new input buffer large enough to accomodate
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.I size
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characters and associated with the given file. When in doubt, use
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.B YY_BUF_SIZE
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for the size.
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.IP -
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.B yy_switch_to_buffer( new_buffer )
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switches the scanner's processing to scan for tokens from
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the given buffer, which must be a YY_BUFFER_STATE.
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.IP -
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.B yy_delete_buffer( buffer )
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deletes the given buffer.
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.SH VALUES AVAILABLE TO THE USER
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.IP -
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.B char *yytext
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holds the text of the current token. It may not be modified.
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.IP -
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.B int yyleng
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holds the length of the current token. It may not be modified.
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.IP -
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.B FILE *yyin
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is the file which by default
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.I flex
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reads from. It may be redefined but doing so only makes sense before
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scanning begins. Changing it in the middle of scanning will have
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unexpected results since
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.I flex
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buffers its input. Once scanning terminates because an end-of-file
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has been seen,
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.B
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void yyrestart( FILE *new_file )
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may be called to point
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.I yyin
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at the new input file.
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.IP -
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.B FILE *yyout
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is the file to which
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.B ECHO
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actions are done. It can be reassigned by the user.
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.IP -
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.B YY_CURRENT_BUFFER
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returns a
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.B YY_BUFFER_STATE
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handle to the current buffer.
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.SH MACROS THE USER CAN REDEFINE
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.IP -
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.B YY_DECL
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controls how the scanning routine is declared.
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By default, it is "int yylex()", or, if prototypes are being
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used, "int yylex(void)". This definition may be changed by redefining
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the "YY_DECL" macro. Note that
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if you give arguments to the scanning routine using a
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K&R-style/non-prototyped function declaration, you must terminate
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the definition with a semi-colon (;).
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.IP -
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The nature of how the scanner
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gets its input can be controlled by redefining the
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.B YY_INPUT
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macro.
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YY_INPUT's calling sequence is "YY_INPUT(buf,result,max_size)". Its
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action is to place up to
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.I max_size
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characters in the character array
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.I buf
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and return in the integer variable
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.I result
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either the
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number of characters read or the constant YY_NULL (0 on Unix systems)
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|
to indicate EOF. The default YY_INPUT reads from the
|
|
global file-pointer "yyin".
|
|
A sample redefinition of YY_INPUT (in the definitions
|
|
section of the input file):
|
|
.nf
|
|
|
|
%{
|
|
#undef YY_INPUT
|
|
#define YY_INPUT(buf,result,max_size) \\
|
|
{ \\
|
|
int c = getchar(); \\
|
|
result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \\
|
|
}
|
|
%}
|
|
|
|
.fi
|
|
.IP -
|
|
When the scanner receives an end-of-file indication from YY_INPUT,
|
|
it then checks the
|
|
.B yywrap()
|
|
function. If
|
|
.B yywrap()
|
|
returns false (zero), then it is assumed that the
|
|
function has gone ahead and set up
|
|
.I yyin
|
|
to point to another input file, and scanning continues. If it returns
|
|
true (non-zero), then the scanner terminates, returning 0 to its
|
|
caller.
|
|
.IP
|
|
The default
|
|
.B yywrap()
|
|
always returns 1. Presently, to redefine it you must first
|
|
"#undef yywrap", as it is currently implemented as a macro. It is
|
|
likely that
|
|
.B yywrap()
|
|
will soon be defined to be a function rather than a macro.
|
|
.IP -
|
|
YY_USER_ACTION
|
|
can be redefined to provide an action
|
|
which is always executed prior to the matched rule's action.
|
|
.IP -
|
|
The macro
|
|
.B YY_USER_INIT
|
|
may be redefined to provide an action which is always executed before
|
|
the first scan.
|
|
.IP -
|
|
In the generated scanner, the actions are all gathered in one large
|
|
switch statement and separated using
|
|
.B YY_BREAK,
|
|
which may be redefined. By default, it is simply a "break", to separate
|
|
each rule's action from the following rule's.
|
|
.SH FILES
|
|
.TP
|
|
.I flex.skel
|
|
skeleton scanner.
|
|
.TP
|
|
.I lex.yy.c
|
|
generated scanner (called
|
|
.I lexyy.c
|
|
on some systems).
|
|
.TP
|
|
.I lex.backtrack
|
|
backtracking information for
|
|
.B -b
|
|
flag (called
|
|
.I lex.bck
|
|
on some systems).
|
|
.TP
|
|
.B -lfl
|
|
library with which to link the scanners.
|
|
.SH "SEE ALSO"
|
|
.LP
|
|
flexdoc(1), lex(1), yacc(1), sed(1), awk(1).
|
|
.LP
|
|
M. E. Lesk and E. Schmidt,
|
|
.I LEX - Lexical Analyzer Generator
|
|
.SH DIAGNOSTICS
|
|
.I reject_used_but_not_detected undefined
|
|
or
|
|
.LP
|
|
.I yymore_used_but_not_detected undefined -
|
|
These errors can occur at compile time. They indicate that the
|
|
scanner uses
|
|
.B REJECT
|
|
or
|
|
.B yymore()
|
|
but that
|
|
.I flex
|
|
failed to notice the fact, meaning that
|
|
.I flex
|
|
scanned the first two sections looking for occurrences of these actions
|
|
and failed to find any, but somehow you snuck some in (via a #include
|
|
file, for example). Make an explicit reference to the action in your
|
|
.I flex
|
|
input file. (Note that previously
|
|
.I flex
|
|
supported a
|
|
.B %used/%unused
|
|
mechanism for dealing with this problem; this feature is still supported
|
|
but now deprecated, and will go away soon unless the author hears from
|
|
people who can argue compellingly that they need it.)
|
|
.LP
|
|
.I flex scanner jammed -
|
|
a scanner compiled with
|
|
.B -s
|
|
has encountered an input string which wasn't matched by
|
|
any of its rules.
|
|
.LP
|
|
.I flex input buffer overflowed -
|
|
a scanner rule matched a string long enough to overflow the
|
|
scanner's internal input buffer (16K bytes - controlled by
|
|
.B YY_BUF_MAX
|
|
in "flex.skel").
|
|
.LP
|
|
.I scanner requires -8 flag -
|
|
Your scanner specification includes recognizing 8-bit characters and
|
|
you did not specify the -8 flag (and your site has not installed flex
|
|
with -8 as the default).
|
|
.LP
|
|
.I
|
|
fatal flex scanner internal error--end of buffer missed -
|
|
This can occur in an scanner which is reentered after a long-jump
|
|
has jumped out (or over) the scanner's activation frame. Before
|
|
reentering the scanner, use:
|
|
.nf
|
|
|
|
yyrestart( yyin );
|
|
|
|
.fi
|
|
.LP
|
|
.I too many %t classes! -
|
|
You managed to put every single character into its own %t class.
|
|
.I flex
|
|
requires that at least one of the classes share characters.
|
|
.SH AUTHOR
|
|
Vern Paxson, with the help of many ideas and much inspiration from
|
|
Van Jacobson. Original version by Jef Poskanzer.
|
|
.LP
|
|
See flexdoc(1) for additional credits and the address to send comments to.
|
|
.SH DEFICIENCIES / BUGS
|
|
.LP
|
|
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.)
|
|
.LP
|
|
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.
|
|
.LP
|
|
Combining trailing context with the special '|' action can result in
|
|
.I fixed
|
|
trailing context being turned into the more expensive
|
|
.I variable
|
|
trailing context. For example, this happens in the following example:
|
|
.nf
|
|
|
|
%%
|
|
abc |
|
|
xyz/def
|
|
|
|
.fi
|
|
.LP
|
|
Use of unput() invalidates yytext and yyleng.
|
|
.LP
|
|
Use of unput() to push back more text than was matched can
|
|
result in the pushed-back text matching a beginning-of-line ('^')
|
|
rule even though it didn't come at the beginning of the line
|
|
(though this is rare!).
|
|
.LP
|
|
Pattern-matching of NUL's is substantially slower than matching other
|
|
characters.
|
|
.LP
|
|
.I flex
|
|
does not generate correct #line directives for code internal
|
|
to the scanner; thus, bugs in
|
|
.I flex.skel
|
|
yield bogus line numbers.
|
|
.LP
|
|
Due to both buffering of input and read-ahead, you cannot intermix
|
|
calls to <stdio.h> routines, such as, for example,
|
|
.B getchar(),
|
|
with
|
|
.I flex
|
|
rules and expect it to work. Call
|
|
.B input()
|
|
instead.
|
|
.LP
|
|
The total table entries listed by the
|
|
.B -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
|
|
.B REJECT,
|
|
and somewhat greater than the number of states if it does.
|
|
.LP
|
|
.B REJECT
|
|
cannot be used with the
|
|
.I -f
|
|
or
|
|
.I -F
|
|
options.
|
|
.LP
|
|
Some of the macros, such as
|
|
.B yywrap(),
|
|
may in the future become functions which live in the
|
|
.B -lfl
|
|
library. This will doubtless break a lot of code, but may be
|
|
required for POSIX-compliance.
|
|
.LP
|
|
The
|
|
.I flex
|
|
internal algorithms need documentation.
|