LEX(1)                     POSIX Programmer's Manual                    LEX(1)



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NAME
       lex -- generate programs for lexical tasks (DEVELOPMENT)

SYNOPSIS
       lex [[-t]] [[-n|-v]] [[file...]]

DESCRIPTION
       The lex utility shall generate C programs to be used in lexical
       processing of character input, and that can be used as an interface to
       yacc.  The C programs shall be generated from lex source code and
       conform to the ISO C standard, without depending on any undefined,
       unspecified, or implementation-defined behavior, except in cases where
       the code is copied directly from the supplied source, or in cases that
       are documented by the implementation. Usually, the lex utility shall
       write the program it generates to the file lex.yy.c; the state of this
       file is unspecified if lex exits with a non-zero exit status. See the
       EXTENDED DESCRIPTION section for a complete description of the lex
       input language.

OPTIONS
       The lex utility shall conform to the Base Definitions volume of
       POSIX.1-2017, Section 12.2, Utility Syntax Guidelines, except for
       Guideline 9.

       The following options shall be supported:

       -n        Suppress the summary of statistics usually written with the
                 -v option. If no table sizes are specified in the lex source
                 code and the -v option is not specified, then -n is implied.

       -t        Write the resulting program to standard output instead of
                 lex.yy.c.

       -v        Write a summary of lex statistics to the standard output.
                 (See the discussion of lex table sizes in Definitions in
                 lex.)  If the -t option is specified and -n is not specified,
                 this report shall be written to standard error. If table
                 sizes are specified in the lex source code, and if the -n
                 option is not specified, the -v option may be enabled.

OPERANDS
       The following operand shall be supported:

       file      A pathname of an input file. If more than one such file is
                 specified, all files shall be concatenated to produce a
                 single lex program. If no file operands are specified, or if
                 a file operand is ''--'', the standard input shall be used.

STDIN
       The standard input shall be used if no file operands are specified, or
       if a file operand is ''--''.  See INPUT FILES.

INPUT FILES
       The input files shall be text files containing lex source code, as
       described in the EXTENDED DESCRIPTION section.

ENVIRONMENT VARIABLES
       The following environment variables shall affect the execution of lex:

       LANG      Provide a default value for the internationalization
                 variables that are unset or null. (See the Base Definitions
                 volume of POSIX.1-2017, Section 8.2, Internationalization
                 Variables for the precedence of internationalization
                 variables used to determine the values of locale categories.)

       LC__ALL    If set to a non-empty string value, override the values of
                 all the other internationalization variables.

       LC__COLLATE
                 Determine the locale for the behavior of ranges, equivalence
                 classes, and multi-character collating elements within
                 regular expressions. If this variable is not set to the POSIX
                 locale, the results are unspecified.

       LC__CTYPE  Determine the locale for the interpretation of sequences of
                 bytes of text data as characters (for example, single-byte as
                 opposed to multi-byte characters in arguments and input
                 files), and the behavior of character classes within regular
                 expressions. If this variable is not set to the POSIX locale,
                 the results are unspecified.

       LC__MESSAGES
                 Determine the locale that should be used to affect the format
                 and contents of diagnostic messages written to standard
                 error.

       NLSPATH   Determine the location of message catalogs for the processing
                 of LC__MESSAGES.

ASYNCHRONOUS EVENTS
       Default.

STDOUT
       If the -t option is specified, the text file of C source code output of
       lex shall be written to standard output.

       If the -t option is not specified:

        *  Implementation-defined informational, error, and warning messages
           concerning the contents of lex source code input shall be written
           to either the standard output or standard error.

        *  If the -v option is specified and the -n option is not specified,
           lex statistics shall also be written to either the standard output
           or standard error, in an implementation-defined format. These
           statistics may also be generated if table sizes are specified with
           a ''%%'' operator in the Definitions section, as long as the -n option
           is not specified.

STDERR
       If the -t option is specified, implementation-defined informational,
       error, and warning messages concerning the contents of lex source code
       input shall be written to the standard error.

       If the -t option is not specified:

        1. Implementation-defined informational, error, and warning messages
           concerning the contents of lex source code input shall be written
           to either the standard output or standard error.

        2. If the -v option is specified and the -n option is not specified,
           lex statistics shall also be written to either the standard output
           or standard error, in an implementation-defined format. These
           statistics may also be generated if table sizes are specified with
           a ''%%'' operator in the Definitions section, as long as the -n option
           is not specified.

OUTPUT FILES
       A text file containing C source code shall be written to lex.yy.c, or
       to the standard output if the -t option is present.

EXTENDED DESCRIPTION
       Each input file shall contain lex source code, which is a table of
       regular expressions with corresponding actions in the form of C program
       fragments.

       When lex.yy.c is compiled and linked with the lex library (using the
       -l ll operand with c99), the resulting program shall read character
       input from the standard input and shall partition it into strings that
       match the given expressions.

       When an expression is matched, these actions shall occur:

        *  The input string that was matched shall be left in yytext as a
           null-terminated string; yytext shall either be an external
           character array or a pointer to a character string. As explained in
           Definitions in lex, the type can be explicitly selected using the
           %array or %pointer declarations, but the default is implementation-
           defined.

        *  The external int yyleng shall be set to the length of the matching
           string.

        *  The expression's corresponding program fragment, or action, shall
           be executed.

       During pattern matching, lex shall search the set of patterns for the
       single longest possible match. Among rules that match the same number
       of characters, the rule given first shall be chosen.

       The general format of lex source shall be:

              Definitions %%%% Rules %%%% UserSubroutines

       The first ""%%%%"" is required to mark the beginning of the rules (regular
       expressions and actions); the second ""%%%%"" is required only if user
       subroutines follow.

       Any line in the Definitions section beginning with a <blank> shall be
       assumed to be a C program fragment and shall be copied to the external
       definition area of the lex.yy.c file. Similarly, anything in the
       Definitions section included between delimiter lines containing only
       ""%%{{"" and ""%%}}"" shall also be copied unchanged to the external definition
       area of the lex.yy.c file.

       Any such input (beginning with a <blank> or within ""%%{{"" and ""%%}}""
       delimiter lines) appearing at the beginning of the Rules section before
       any rules are specified shall be written to lex.yy.c after the
       declarations of variables for the yylex() function and before the first
       line of code in yylex().  Thus, user variables local to yylex() can be
       declared here, as well as application code to execute upon entry to
       yylex().

       The action taken by lex when encountering any input beginning with a
       <blank> or within ""%%{{"" and ""%%}}"" delimiter lines appearing in the Rules
       section but coming after one or more rules is undefined. The presence
       of such input may result in an erroneous definition of the yylex()
       function.

       C-language code in the input shall not contain C-language trigraphs.
       The C-language code within ""%%{{"" and ""%%}}"" delimiter lines shall not
       contain any lines consisting only of ""%%}}"", or only of ""%%%%"".

   Definitions in lex
       Definitions appear before the first ""%%%%"" delimiter. Any line in this
       section not contained between ""%%{{"" and ""%%}}"" lines and not beginning
       with a <blank> shall be assumed to define a lex substitution string.
       The format of these lines shall be:


           name substitute

       If a name does not meet the requirements for identifiers in the ISO C
       standard, the result is undefined. The string substitute shall replace
       the string {name} when it is used in a rule. The name string shall be
       recognized in this context only when the braces are provided and when
       it does not appear within a bracket expression or within double-quotes.

       In the Definitions section, any line beginning with a <percent-sign>
       (''%%'') character and followed by an alphanumeric word beginning with
       either 's' or 'S' shall define a set of start conditions. Any line
       beginning with a ''%%'' followed by a word beginning with either 'x' or
       'X' shall define a set of exclusive start conditions. When the
       generated scanner is in a %s state, patterns with no state specified
       shall be also active; in a %x state, such patterns shall not be active.
       The rest of the line, after the first word, shall be considered to be
       one or more <blank>-separated names of start conditions. Start
       condition names shall be constructed in the same way as definition
       names. Start conditions can be used to restrict the matching of regular
       expressions to one or more states as described in Regular Expressions
       in lex.

       Implementations shall accept either of the following two mutually-
       exclusive declarations in the Definitions section:

       %array    Declare the type of yytext to be a null-terminated character
                 array.

       %pointer  Declare the type of yytext to be a pointer to a null-
                 terminated character string.

       The default type of yytext is implementation-defined. If an application
       refers to yytext outside of the scanner source file (that is, via an
       extern), the application shall include the appropriate %array or
       %pointer declaration in the scanner source file.

       Implementations shall accept declarations in the Definitions section
       for setting certain internal table sizes. The declarations are shown in
       the following table.

                        Table: Table Size Declarations in lex

         +------------+------------------------------------+---------------+
         |Declaration |            Description             | Minimum Value |
         +------------+------------------------------------+---------------+
         |%pp _n        | Number of positions                |     2500      |
         |%nn _n        | Number of states                   |      500      |
         |%aa _n        | Number of transitions              |     2000      |
         |%ee _n        | Number of parse tree nodes         |     1000      |
         |%kk _n        | Number of packed character classes |     1000      |
         |%oo _n        | Size of the output array           |     3000      |
         +------------+------------------------------------+---------------+
       In the table, _n represents a positive decimal integer, preceded by one
       or more <blank> characters. The exact meaning of these table size
       numbers is implementation-defined. The implementation shall document
       how these numbers affect the lex utility and how they are related to
       any output that may be generated by the implementation should
       limitations be encountered during the execution of lex.  It shall be
       possible to determine from this output which of the table size values
       needs to be modified to permit lex to successfully generate tables for
       the input language. The values in the column Minimum Value represent
       the lowest values conforming implementations shall provide.

   Rules in lex
       The rules in lex source files are a table in which the left column
       contains regular expressions and the right column contains actions (C
       program fragments) to be executed when the expressions are recognized.


           ERE action
           ERE action
           ...

       The extended regular expression (ERE) portion of a row shall be
       separated from action by one or more <blank> characters. A regular
       expression containing <blank> characters shall be recognized under one
       of the following conditions:

        *  The entire expression appears within double-quotes.

        *  The <blank> characters appear within double-quotes or square
           brackets.

        *  Each <blank> is preceded by a <backslash> character.

   User Subroutines in lex
       Anything in the user subroutines section shall be copied to lex.yy.c
       following yylex().

   Regular Expressions in lex
       The lex utility shall support the set of extended regular expressions
       (see the Base Definitions volume of POSIX.1-2017, Section 9.4, Extended
       Regular Expressions), with the following additions and exceptions to
       the syntax:

       "..."     Any string enclosed in double-quotes shall represent the
                 characters within the double-quotes as themselves, except
                 that <backslash>-escapes (which appear in the following
                 table) shall be recognized. Any <backslash>-escape sequence
                 shall be terminated by the closing quote. For example,
                 ""\\0011""""11"" represents a single string: the octal value 1
                 followed by the character ''11''.

       <state>_r, <state1,state2,...>_r
                 The regular expression _r shall be matched only when the
                 program is in one of the start conditions indicated by state,
                 state1, and so on; see Actions in lex.  (As an exception to
                 the typographical conventions of the rest of this volume of
                 POSIX.1-2017, in this case <state> does not represent a
                 metavariable, but the literal angle-bracket characters
                 surrounding a symbol.) The start condition shall be
                 recognized as such only at the beginning of a regular
                 expression.

       _r/_x       The regular expression _r shall be matched only if it is
                 followed by an occurrence of regular expression _x (_x is the
                 instance of trailing context, further defined below). The
                 token returned in yytext shall only match _r.  If the trailing
                 portion of _r matches the beginning of _x, the result is
                 unspecified. The _r expression cannot include further trailing
                 context or the ''$$'' (match-end-of-line) operator; _x cannot
                 include the ''^^'' (match-beginning-of-line) operator, nor
                 trailing context, nor the ''$$'' operator. That is, only one
                 occurrence of trailing context is allowed in a lex regular
                 expression, and the ''^^'' operator only can be used at the
                 beginning of such an expression.

       {name}    When name is one of the substitution symbols from the
                 Definitions section, the string, including the enclosing
                 braces, shall be replaced by the substitute value. The
                 substitute value shall be treated in the extended regular
                 expression as if it were enclosed in parentheses. No
                 substitution shall occur if {name} occurs within a bracket
                 expression or within double-quotes.

       Within an ERE, a <backslash> character shall be considered to begin an
       escape sequence as specified in the table in the Base Definitions
       volume of POSIX.1-2017, Chapter _5, File Format Notation (''\\\\'', '\a',
       '\b', '\f', '\n', '\r', '\t', '\v').  In addition, the escape sequences
       in the following table shall be recognized.

       A literal <newline> cannot occur within an ERE; the escape sequence
       '\n' can be used to represent a <newline>.  A <newline> shall not be
       matched by a period operator.

                           Table: Escape Sequences in lex

          +---------+--------------------------+--------------------------+
          | Escape  |                          |                          |
          |Sequence |       Description        |         Meaning          |
          +---------+--------------------------+--------------------------+
          |\digits  | A <backslash> character  | The character whose      |
          |         | followed by the longest  | encoding is represented  |
          |         | sequence of one, two, or | by the one, two, or      |
          |         | three octal-digit        | three-digit octal        |
          |         | characters (01234567).   | integer. Multi-byte      |
          |         | If all of the digits are | characters require       |
          |         | 0 (that is,              | multiple, concatenated   |
          |         | representation of the    | escape sequences of this |
          |         | NUL character), the      | type, including the      |
          |         | behavior is undefined.   | leading <backslash> for  |
          |         |                          | each byte.               |
          +---------+--------------------------+--------------------------+
          |\x_d_i_g_i_t_s | A <backslash> character  | The character whose      |
          |         | followed by the longest  | encoding is represented  |
          |         | sequence of hexadecimal- | by the hexadecimal       |
          |         | digit characters         | integer.                 |
          |         | (01234567abcdefABCDEF).  |                          |
          |         | If all of the digits are |                          |
          |         | 0 (that is,              |                          |
          |         | representation of the    |                          |
          |         | NUL character), the      |                          |
          |         | behavior is undefined.   |                          |
          +---------+--------------------------+--------------------------+
          |\c       | A <backslash> character  | The character 'c',       |
          |         | followed by any          | unchanged.               |
          |         | character not described  |                          |
          |         | in this table or in the  |                          |
          |         | table in the Base        |                          |
          |         | Definitions volume of    |                          |
          |         | POSIX.1-2017, Chapter _5, |                          |
          |         | File Format Notation     |                          |
          |         | (''\\\\'', '\a', '\b', '\f', |                          |
          |         | '\n', '\r', '\t', '\v'). |                          |
          +---------+--------------------------+--------------------------+
       Note:     If a '\x' sequence needs to be immediately followed by a
                 hexadecimal digit character, a sequence such as "\x1""1" can
                 be used, which represents a character containing the value 1,
                 followed by the character ''11''.

       The order of precedence given to extended regular expressions for lex
       differs from that specified in the Base Definitions volume of
       POSIX.1-2017, Section 9.4, Extended Regular Expressions.  The order of
       precedence for lex shall be as shown in the following table, from high
       to low.

       Note:     The escaped characters entry is not meant to imply that these
                 are operators, but they are included in the table to show
                 their relationships to the true operators. The start
                 condition, trailing context, and anchoring notations have
                 been omitted from the table because of the placement
                 restrictions described in this section; they can only appear
                 at the beginning or ending of an ERE.

                            Table: ERE Precedence in lex

             +----------------------------------+----------------------+
             |   Extended Regular Expression    |      Precedence      |
             +----------------------------------+----------------------+
             |collation-related bracket symbols | [= =]  [: :]  [. .]  |
             |escaped characters                | \<special character> |
             |bracket expression                | [ ]                  |
             |quoting                           | "..."                |
             |grouping                          | ( )                  |
             |definition                        | {name}               |
             |single-character RE duplication   | * + ?                |
             |concatenation                     |                      |
             |interval expression               | {m,n}                |
             |alternation                       | |                    |
             +----------------------------------+----------------------+
       The ERE anchoring operators ''^^'' and ''$$'' do not appear in the table.
       With lex regular expressions, these operators are restricted in their
       use: the ''^^'' operator can only be used at the beginning of an entire
       regular expression, and the ''$$'' operator only at the end. The operators
       apply to the entire regular expression. Thus, for example, the pattern
       "(^abc)|(def$)" is undefined; it can instead be written as two separate
       rules, one with the regular expression "^abc" and one with "def$",
       which share a common action via the special ''||'' action (see below). If
       the pattern were written "^abc|def$", it would match either "abc" or
       "def" on a line by itself.

       Unlike the general ERE rules, embedded anchoring is not allowed by most
       historical lex implementations. An example of embedded anchoring would
       be for patterns such as ""((^^|| )foo( ||$$))"" to match "foo" when it exists
       as a complete word. This functionality can be obtained using existing
       lex features:


           ^foo/[ \n]      |
           " foo"/[ \n]    /* Found foo as a separate word. */

       Note also that ''$$'' is a form of trailing context (it is equivalent to
       "/\n") and as such cannot be used with regular expressions containing
       another instance of the operator (see the preceding discussion of
       trailing context).

       The additional regular expressions trailing-context operator ''//'' can be
       used as an ordinary character if presented within double-quotes, ""//"";
       preceded by a <backslash>, ""\\//""; or within a bracket expression, ""[[//]]"".
       The start-condition ''<<'' and ''>>'' operators shall be special only in a
       start condition at the beginning of a regular expression; elsewhere in
       the regular expression they shall be treated as ordinary characters.

   Actions in lex
       The action to be taken when an ERE is matched can be a C program
       fragment or the special actions described below; the program fragment
       can contain one or more C statements, and can also include special
       actions. The empty C statement '';;'' shall be a valid action; any string
       in the lex.yy.c input that matches the pattern portion of such a rule
       is effectively ignored or skipped. However, the absence of an action
       shall not be valid, and the action lex takes in such a condition is
       undefined.

       The specification for an action, including C statements and special
       actions, can extend across several lines if enclosed in braces:


           ERE <one or more blanks> { program statement
                                      program statement }

       The program statements shall not contain unbalanced curly brace
       preprocessing tokens.

       The default action when a string in the input to a lex.yy.c program is
       not matched by any expression shall be to copy the string to the
       output. Because the default behavior of a program generated by lex is
       to read the input and copy it to the output, a minimal lex source
       program that has just ""%%%%"" shall generate a C program that simply
       copies the input to the output unchanged.

       Four special actions shall be available:


           |   ECHO;   REJECT;   BEGIN

       |         The action ''||'' means that the action for the next rule is the
                 action for this rule.  Unlike the other three actions, ''||''
                 cannot be enclosed in braces or be <semicolon>-terminated;
                 the application shall ensure that it is specified alone, with
                 no other actions.

       ECHO;     Write the contents of the string yytext on the output.

       REJECT;   Usually only a single expression is matched by a given string
                 in the input.  REJECT means ``continue to the next expression
                 that matches the current input'', and shall cause whatever
                 rule was the second choice after the current rule to be
                 executed for the same input. Thus, multiple rules can be
                 matched and executed for one input string or overlapping
                 input strings. For example, given the regular expressions
                 "xyz" and "xy" and the input "xyz", usually only the regular
                 expression "xyz" would match. The next attempted match would
                 start after z.  If the last action in the "xyz" rule is
                 REJECT, both this rule and the "xy" rule would be executed.
                 The REJECT action may be implemented in such a fashion that
                 flow of control does not continue after it, as if it were
                 equivalent to a goto to another part of yylex().  The use of
                 REJECT may result in somewhat larger and slower scanners.

       BEGIN     The action:


                     BEGIN newstate;

                 switches the state (start condition) to newstate.  If the
                 string newstate has not been declared previously as a start
                 condition in the Definitions section, the results are
                 unspecified. The initial state is indicated by the digit ''00''
                 or the token INITIAL.

       The functions or macros described below are accessible to user code
       included in the lex input. It is unspecified whether they appear in the
       C code output of lex, or are accessible only through the -l ll operand
       to c99 (the lex library).

       int yylex(void)
             Performs lexical analysis on the input; this is the primary
             function generated by the lex utility. The function shall return
             zero when the end of input is reached; otherwise, it shall return
             non-zero values (tokens) determined by the actions that are
             selected.

       int yymore(void)
             When called, indicates that when the next input string is
             recognized, it is to be appended to the current value of yytext
             rather than replacing it; the value in yyleng shall be adjusted
             accordingly.

       int yyless(int _n)
             Retains _n initial characters in yytext, NUL-terminated, and
             treats the remaining characters as if they had not been read; the
             value in yyleng shall be adjusted accordingly.

       int input(void)
             Returns the next character from the input, or zero on end-of-
             file. It shall obtain input from the stream pointer yyin,
             although possibly via an intermediate buffer. Thus, once scanning
             has begun, the effect of altering the value of yyin is undefined.
             The character read shall be removed from the input stream of the
             scanner without any processing by the scanner.

       int unput(int _c)
             Returns the character 'c' to the input; yytext and yyleng are
             undefined until the next expression is matched. The result of
             using unput() for more characters than have been input is
             unspecified.

       The following functions shall appear only in the lex library accessible
       through the -l ll operand; they can therefore be redefined by a
       conforming application:

       int yywrap(void)
             Called by yylex() at end-of-file; the default yywrap() shall
             always return 1. If the application requires yylex() to continue
             processing with another source of input, then the application can
             include a function yywrap(), which associates another file with
             the external variable FILE ** yyin and shall return a value of
             zero.

       int main(int argc, char **argv[])
             Calls yylex() to perform lexical analysis, then exits. The user
             code can contain main() to perform application-specific
             operations, calling yylex() as applicable.

       Except for input(), unput(), and main(), all external and static names
       generated by lex shall begin with the prefix yy or YYYY.

EXIT STATUS
       The following exit values shall be returned:

        0    Successful completion.

       >0    An error occurred.

CONSEQUENCES OF ERRORS
       Default.

       The following sections are informative.

APPLICATION USAGE
       Conforming applications are warned that in the Rules section, an ERE
       without an action is not acceptable, but need not be detected as
       erroneous by lex.  This may result in compilation or runtime errors.

       The purpose of input() is to take characters off the input stream and
       discard them as far as the lexical analysis is concerned. A common use
       is to discard the body of a comment once the beginning of a comment is
       recognized.

       The lex utility is not fully internationalized in its treatment of
       regular expressions in the lex source code or generated lexical
       analyzer. It would seem desirable to have the lexical analyzer
       interpret the regular expressions given in the lex source according to
       the environment specified when the lexical analyzer is executed, but
       this is not possible with the current lex technology. Furthermore, the
       very nature of the lexical analyzers produced by lex must be closely
       tied to the lexical requirements of the input language being described,
       which is frequently locale-specific anyway. (For example, writing an
       analyzer that is used for French text is not automatically useful for
       processing other languages.)

EXAMPLES
       The following is an example of a lex program that implements a
       rudimentary scanner for a Pascal-like syntax:


           %{
           /* Need this for the call to atof() below. */
           #include <math.h>
           /* Need this for printf(), fopen(), and stdin below. */
           #include <stdio.h>
           %}

           DIGIT    [0-9]
           ID       [a-z][a-z0-9]*

           %%

           {DIGIT}+ {
               printf("An integer: %s (%d)\n", yytext,
                   atoi(yytext));
               }

           {DIGIT}+"."{DIGIT}*        {
               printf("A float: %s (%g)\n", yytext,
                   atof(yytext));
               }

           if|then|begin|end|procedure|function        {
               printf("A keyword: %s\n", yytext);
               }

           {ID}    printf("An identifier: %s\n", yytext);

           "+"|"-"|"*"|"/"        printf("An operator: %s\n", yytext);

           "{"[^}\n]*"}"    /* Eat up one-line comments. */

           [ \t\n]+        /* Eat up white space. */

           .  printf("Unrecognized character: %s\n", yytext);

           %%

           int main(int argc, char *argv[])
           {
               ++argv, --argc;  /* Skip over program name. */
               if (argc > 0)
                   yyin = fopen(argv[0], "r");
               else
                   yyin = stdin;

               yylex();
           }

RATIONALE
       Even though the -c option and references to the C language are retained
       in this description, lex may be generalized to other languages, as was
       done at one time for EFL, the Extended FORTRAN Language. Since the lex
       input specification is essentially language-independent, versions of
       this utility could be written to produce Ada, Modula-2, or Pascal code,
       and there are known historical implementations that do so.

       The current description of lex bypasses the issue of dealing with
       internationalized EREs in the lex source code or generated lexical
       analyzer. If it follows the model used by awk (the source code is
       assumed to be presented in the POSIX locale, but input and output are
       in the locale specified by the environment variables), then the tables
       in the lexical analyzer produced by lex would interpret EREs specified
       in the lex source in terms of the environment variables specified when
       lex was executed. The desired effect would be to have the lexical
       analyzer interpret the EREs given in the lex source according to the
       environment specified when the lexical analyzer is executed, but this
       is not possible with the current lex technology.

       The description of octal and hexadecimal-digit escape sequences agrees
       with the ISO C standard use of escape sequences.

       Earlier versions of this standard allowed for implementations with
       bytes other than eight bits, but this has been modified in this
       version.

       There is no detailed output format specification. The observed behavior
       of lex under four different historical implementations was that none of
       these implementations consistently reported the line numbers for error
       and warning messages. Furthermore, there was a desire that lex be
       allowed to output additional diagnostic messages. Leaving message
       formats unspecified avoids these formatting questions and problems with
       internationalization.

       Although the %x specifier for exclusive start conditions is not
       historical practice, it is believed to be a minor change to historical
       implementations and greatly enhances the usability of lex programs
       since it permits an application to obtain the expected functionality
       with fewer statements.

       The %array and %pointer declarations were added as a compromise between
       historical systems.  The System V-based lex copies the matched text to
       a yytext array. The flex program, supported in BSD and GNU systems,
       uses a pointer. In the latter case, significant performance
       improvements are available for some scanners. Most historical programs
       should require no change in porting from one system to another because
       the string being referenced is null-terminated in both cases. (The
       method used by flex in its case is to null-terminate the token in place
       by remembering the character that used to come right after the token
       and replacing it before continuing on to the next scan.) Multi-file
       programs with external references to yytext outside the scanner source
       file should continue to operate on their historical systems, but would
       require one of the new declarations to be considered strictly portable.

       The description of EREs avoids unnecessary duplication of ERE details
       because their meanings within a lex ERE are the same as that for the
       ERE in this volume of POSIX.1-2017.

       The reason for the undefined condition associated with text beginning
       with a <blank> or within ""%%{{"" and ""%%}}"" delimiter lines appearing in the
       Rules section is historical practice. Both the BSD and System V lex
       copy the indented (or enclosed) input in the Rules section (except at
       the beginning) to unreachable areas of the yylex() function (the code
       is written directly after a break statement). In some cases, the System
       V lex generates an error message or a syntax error, depending on the
       form of indented input.

       The intention in breaking the list of functions into those that may
       appear in lex.yy.c versus those that only appear in libl.a is that only
       those functions in libl.a can be reliably redefined by a conforming
       application.

       The descriptions of standard output and standard error are somewhat
       complicated because historical lex implementations chose to issue
       diagnostic messages to standard output (unless -t was given).
       POSIX.1-2008 allows this behavior, but leaves an opening for the more
       expected behavior of using standard error for diagnostics.  Also, the
       System V behavior of writing the statistics when any table sizes are
       given is allowed, while BSD-derived systems can avoid it. The
       programmer can always precisely obtain the desired results by using
       either the -t or -n options.

       The OPERANDS section does not mention the use of -- as a synonym for
       standard input; not all historical implementations support such usage
       for any of the file operands.

       A description of the translation table was deleted from early proposals
       because of its relatively low usage in historical applications.

       The change to the definition of the input() function that allows
       buffering of input presents the opportunity for major performance gains
       in some applications.

       The following examples clarify the differences between lex regular
       expressions and regular expressions appearing elsewhere in this volume
       of POSIX.1-2017. For regular expressions of the form "r/x", the string
       matching _r is always returned; confusion may arise when the beginning
       of _x matches the trailing portion of _r.  For example, given the regular
       expression "a*b/cc" and the input "aaabcc", yytext would contain the
       string "aaab" on this match. But given the regular expression "x*/xy"
       and the input "xxxy", the token xxxxxx, not xxxx, is returned by some
       implementations because xxx matches "x*".

       In the rule "ab*/bc", the "b*" at the end of _r extends _r's match into
       the beginning of the trailing context, so the result is unspecified. If
       this rule were "ab/bc", however, the rule matches the text "ab" when it
       is followed by the text "bc".  In this latter case, the matching of _r
       cannot extend into the beginning of _x, so the result is specified.

FUTURE DIRECTIONS
       None.

SEE ALSO
       c99, ed, yacc

       The Base Definitions volume of POSIX.1-2017, Chapter _5, File Format
       Notation, Chapter _8, Environment Variables, Chapter _9, Regular
       Expressions, Section 12.2, Utility Syntax Guidelines

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                               LEX(1)
