pt::peg_language(n)              Parser Tools              pt::peg_language(n)



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NAME
       pt::peg_language - PEG Language Tutorial

SYNOPSIS
       package require Tcl  8.5

_________________________________________________________________

DESCRIPTION
       Are you lost ?  Do you have trouble understanding this document ?  In
       that case please read the overview provided by the Introduction to
       Parser Tools. This document is the entrypoint to the whole system the
       current package is a part of.

       Welcome to the tutorial / introduction for the PEG Specification
       Language.  If you are already familiar with the language we are about
       to discuss, and only wish to refresh your memory you can, of course,
       skip ahead to the aforementioned section and just read the full formal
       specification.

WHAT IS IT?
       peg, a language for the specification of parsing expression grammars is
       meant to be human readable, and writable as well, yet strict enough to
       allow its processing by machine. Like any computer language. It was
       defined to make writing the specification of a grammar easy, something
       the other formats found in the Parser Tools do not lend themselves too.

THE ELEMENTS OF THE LANGUAGE
   BASIC STRUCTURE
       The general outline of a textual PEG is


              PEG <<name>> (<<start-expression>>)
                 <<rules>>
              END;

       Note: We are using text in double angle-brackets as place-holders for
       things not yet explained.

   NAMES
       Names are mostly used to identify the nonterminal symbols of the
       grammar, i.e. that which occurs on the left-hand side of a <rule>.  The
       exception to that is the name given after the keyword PEG (see previous
       section), which is the name of the whole grammar itself.

       The structure of a name is simple:

       [1]    It begins with a letter, underscore, or colon, followed by

       [2]    zero or more letters, digits, underscores, or colons.

       Or, in formal textual notation:


                  ([_:] / <alpha>) ([_:] / <alnum>)*

       Examples of names:


                  Hello
                  ::world
                  _:submarine55_

       Examples of text which are not names:


                  12
                  .bogus
                  0wrong
                  @location


   RULES
       The main body of the text of a grammar specification is taken up by the
       rules. Each rule defines the sentence structure of one nonterminal
       symbol. Their basic structure is


                   <<name>>  <-  <<expression>> ;

       The <name> specifies the nonterminal symbol to be defined, the
       <expression> after the arrow (<-) then declares its structure.

       Note that each rule ends in a single semicolon, even the last.  I.e.
       the semicolon is a rule terminator, not a separator.

       We can have as many rules as we like, as long as we define each
       nonterminal symbol at most once, and have at least one rule for each
       nonterminal symbol which occured in an expression, i.e. in either the
       start expression of the grammar, or the right-hande side of a rule.

   EXPRESSIONS
       The parsing expressions are the meat of any specification. They declare
       the structure of the whole document (<<start-expression>>), and of all
       nonterminal symbols.

       All expressions are made up out of atomic expressions and operators
       combining them. We have operators for choosing between alternatives,
       repetition of parts, and for look-ahead constraints. There is no
       explicit operator for the sequencing (also known as concatenation) of
       parts however. This is specified by simply placing the parts adjacent
       to each other.

       Here are the operators, from highest to lowest priority (i.e. strength
       of binding):


                  # Binary operators.

                  <<expression-1>>     <<expression-2>>  # sequence. parse 1, then 2.
                  <<expression-1>>  /  <<expression-2>>  # alternative. try to parse 1, and parse 2 if 1 failed to parse.

                  # Prefix operators. Lookahead constraints. Same priority.

                  & <<expression>>  # Parse expression, ok on successful parse.
                  ! <<expression>>  # Ditto, except ok on failure to parse.

                  # Suffix operators. Repetition. Same priority.

                  <<expression>> ?  # Parse expression none, or once (repeat 0 or 1).
                  <<expression>> *  # Parse expression zero or more times.
                  <<expression>> +  # Parse expression one or more times.

                  # Expression nesting

                  ( <<expression>> ) # Put an expression in parens to change its priority.

       With this we can now deconstruct the formal expression for names given
       in section Names:


                  ([_:] / <alpha>) ([_:] / <alnum>)*

       It is a sequence of two parts,

                  [_:] / <alpha>
       and

                  ([_:] / <alnum>)*
       The parentheses around the parts kept their inner alternatives bound
       together against the normally higher priority of the sequence. Each of
       the two parts is an alternative, with the second part additionally
       repeated zero or more times, leaving us with the three atomic
       expressions


                  [_:]
                  <alpha>
                  <alnum>

       And atomic expressions are our next topic. They fall into three
       classes:

       [1]    names, i.e. nonterminal symbols,

       [2]    string literals, and

       [3]    character classes.

       Names we know about already, or see section Names for a refresher.

       String literals are simple. They are delimited by (i.e. start and end
       with) either a single or double-apostroph, and in between the
       delimiters we can have any character but the delimiter itself. They can
       be empty as well. Examples of strings are


                  ''
                  ""
                  'hello'
                  "umbra"
                  "'"
                  '"'

       The last two examples show how to place any of the delimiters into a
       string.

       For the last, but not least of our atomic expressions, character
       classes, we have a number of predefined classes, shown below, and the
       ability to construct or own. The predefined classes are:


                  <alnum>    # Any unicode alphabet or digit character (string is alnum).
                  <alpha>    # Any unicode alphabet character (string is alpha).
                  <ascii>    # Any unicode character below codepoint 0x80 (string is ascii).
                  <control>  # Any unicode control character (string is control).
                  <ddigit>   # The digit characters [0-9].
                  <digit>    # Any unicode digit character (string is digit).
                  <graph>    # Any unicode printing character, except space (string is graph).
                  <lower>    # Any unicode lower-case alphabet character (string is lower).
                  <print>    # Any unicode printing character, incl. space (string is print).
                  <punct>    # Any unicode punctuation character (string is punct).
                  <space>    # Any unicode space character (string is space).
                  <upper>    # Any unicode upper-case alphabet character (string is upper).
                  <wordchar> # Any unicode word character (string is wordchar).
                  <xdigit>   # The hexadecimal digit characters [0-9a-fA-F].
                  .          # Any character, except end of input.

       And the syntax of custom-defined character classes is


                  [ <<range>>* ]

       where each range is either a single character, or of the form


                 <<character>> - <character>>

       Examples for character classes we have seen already in the course of
       this introduction are


                  [_:]
                  [0-9]
                  [0-9a-fA-F]

       We are nearly done with expressions. The only piece left is to tell how
       the characters in character classes and string literals are specified.

       Basically characters in the input stand for themselves, and in addition
       to that we several types of escape syntax to to repesent control
       characters, or characters outside of the encoding the text is in.

       All the escaped forms are started with a backslash character ('\',
       unicode codepoint 0x5C). This is then followed by a series of octal
       digits, or 'u' and hexedecimal digits, or a regular character from a
       fixed set for various control characters. Some examples:


                  \n \r \t \' \" \[ \] \\ #
                  \000 up to \277         # octal escape, all ascii character, leading 0's can be removed.
                  \u2CA7                  # hexadecimal escape, all unicode characters.
                  #                       # Here 2ca7 <=> Koptic Small Letter Tau


   WHITESPACE AND COMMENTS
       One issue not touched upon so far is whitespace and comments.

       Whitespace is any unicode space character, i.e. anything in the
       character class <space>, and comments. The latter are sequences of
       characters starting with a '#' (hash, unicode codepoint 0x23) and
       ending at the next end-of-line.

       Whitespace can be freely used between all syntactical elements of a
       grammar specification. It cannot be used inside of syntactical
       elements, like names, string literals, predefined character classes,
       etc.

   NONTERMINAL ATTRIBUTES
       Lastly, a more advanced topic. In the section Rules we gave the
       structure of a rule as


                   <<name>>  <-  <<expression>> ;

       This is not quite true. It is possible to associate a semantic mode
       with the nonterminal in the rule, by writing it before the name,
       separated from it by a colon, i.e. writing


                  <<mode>> : <<name>>  <-  <<expression>> ;

       is also allowed. This mode is optional. The known modes and their
       meanings are:

       value  The semantic value of the nonterminal symbol is an abstract
              syntax tree consisting of a single node node for the nonterminal
              itself, which has the ASTs of the symbol's right hand side as
              its children.

       leaf   The semantic value of the nonterminal symbol is an abstract
              syntax tree consisting of a single node node for the
              nonterminal, without any children. Any ASTs generated by the
              symbol's right hand side are discarded.

       void   The nonterminal has no semantic value. Any ASTs generated by the
              symbol's right hand side are discarded (as well).

       Of these three modes only leaf and void can be specified directly.
       value is implicitly specified by the absence of a mode before the
       nonterminal.

       Now, with all the above under our belt it should be possible to not
       only read, but understand the formal specification of the text
       representation shown in the next section, written in itself.

PEG SPECIFICATION LANGUAGE
       peg, a language for the specification of parsing expression grammars is
       meant to be human readable, and writable as well, yet strict enough to
       allow its processing by machine. Like any computer language. It was
       defined to make writing the specification of a grammar easy, something
       the other formats found in the Parser Tools do not lend themselves too.

       It is formally specified by the grammar shown below, written in itself.
       For a tutorial / introduction to the language please go and read the
       PEG Language Tutorial.

              PEG pe-grammar-for-peg (Grammar)

                # --------------------------------------------------------------------
                      # Syntactical constructs

                      Grammar         <- WHITESPACE Header Definition* Final EOF ;

                      Header          <- PEG Identifier StartExpr ;
                      Definition      <- Attribute? Identifier IS Expression SEMICOLON ;
                      Attribute       <- (VOID / LEAF) COLON ;
                      Expression      <- Sequence (SLASH Sequence)* ;
                      Sequence        <- Prefix+ ;
                      Prefix          <- (AND / NOT)? Suffix ;
                      Suffix          <- Primary (QUESTION / STAR / PLUS)? ;
                      Primary         <- ALNUM / ALPHA / ASCII / CONTROL / DDIGIT / DIGIT
                                      /  GRAPH / LOWER / PRINTABLE / PUNCT / SPACE / UPPER
                                      /  WORDCHAR / XDIGIT
                                      / Identifier
                                      /  OPEN Expression CLOSE
                                      /  Literal
                                      /  Class
                                      /  DOT
                                      ;
                      Literal         <- APOSTROPH  (!APOSTROPH  Char)* APOSTROPH  WHITESPACE
                                      /  DAPOSTROPH (!DAPOSTROPH Char)* DAPOSTROPH WHITESPACE ;
                      Class           <- OPENB (!CLOSEB Range)* CLOSEB WHITESPACE ;
                      Range           <- Char TO Char / Char ;

                      StartExpr       <- OPEN Expression CLOSE ;
              void:   Final           <- END SEMICOLON WHITESPACE ;

                      # --------------------------------------------------------------------
                      # Lexing constructs

                      Identifier      <- Ident WHITESPACE ;
              leaf:   Ident           <- ('_' / ':' / <alpha>) ('_' / ':' / <alnum>)* ;
                      Char            <- CharSpecial / CharOctalFull / CharOctalPart
                                      /  CharUnicode / CharUnescaped
                                      ;

              leaf:   CharSpecial     <- "\\" [nrt'"\[\]\\] ;
              leaf:   CharOctalFull   <- "\\" [0-2][0-7][0-7] ;
              leaf:   CharOctalPart   <- "\\" [0-7][0-7]? ;
              leaf:   CharUnicode     <- "\\" 'u' HexDigit (HexDigit (HexDigit HexDigit?)?)? ;
              leaf:   CharUnescaped   <- !"\\" . ;

              void:   HexDigit        <- [0-9a-fA-F] ;

              void:   TO              <- '-'           ;
              void:   OPENB           <- "["           ;
              void:   CLOSEB          <- "]"           ;
              void:   APOSTROPH       <- "'"           ;
              void:   DAPOSTROPH      <- '"'           ;
              void:   PEG             <- "PEG"   WHITESPACE ;
              void:   IS              <- "<-"    WHITESPACE ;
              leaf:   VOID            <- "void"  WHITESPACE ; # Implies that definition has no semantic value.
              leaf:   LEAF            <- "leaf"  WHITESPACE ; # Implies that definition has no terminals.
              void:   END             <- "END"   WHITESPACE ;
              void:   SEMICOLON       <- ";"     WHITESPACE ;
              void:   COLON           <- ":"     WHITESPACE ;
              void:   SLASH           <- "/"     WHITESPACE ;
              leaf:   AND             <- "&"     WHITESPACE ;
              leaf:   NOT             <- "!"     WHITESPACE ;
              leaf:   QUESTION        <- "?"     WHITESPACE ;
              leaf:   STAR            <- "*"     WHITESPACE ;
              leaf:   PLUS            <- "+"     WHITESPACE ;
              void:   OPEN            <- "("     WHITESPACE ;
              void:   CLOSE           <- ")"     WHITESPACE ;
              leaf:   DOT             <- "."     WHITESPACE ;

              leaf:   ALNUM           <- "<alnum>"    WHITESPACE ;
              leaf:   ALPHA           <- "<alpha>"    WHITESPACE ;
              leaf:   ASCII           <- "<ascii>"    WHITESPACE ;
              leaf:   CONTROL         <- "<control>"  WHITESPACE ;
              leaf:   DDIGIT          <- "<ddigit>"   WHITESPACE ;
              leaf:   DIGIT           <- "<digit>"    WHITESPACE ;
              leaf:   GRAPH           <- "<graph>"    WHITESPACE ;
              leaf:   LOWER           <- "<lower>"    WHITESPACE ;
              leaf:   PRINTABLE       <- "<print>"    WHITESPACE ;
              leaf:   PUNCT           <- "<punct>"    WHITESPACE ;
              leaf:   SPACE           <- "<space>"    WHITESPACE ;
              leaf:   UPPER           <- "<upper>"    WHITESPACE ;
              leaf:   WORDCHAR        <- "<wordchar>" WHITESPACE ;
              leaf:   XDIGIT          <- "<xdigit>"   WHITESPACE ;

              void:   WHITESPACE      <- (" " / "\t" / EOL / COMMENT)* ;
              void:   COMMENT         <- '#' (!EOL .)* EOL ;
              void:   EOL             <- "\n\r" / "\n" / "\r" ;
              void:   EOF             <- !. ;

                      # --------------------------------------------------------------------
              END;


   EXAMPLE
       Our example specifies the grammar for a basic 4-operation calculator.

              PEG calculator (Expression)
                  Digit      <- '0'/'1'/'2'/'3'/'4'/'5'/'6'/'7'/'8'/'9'       ;
                  Sign       <- '-' / '+'                                     ;
                  Number     <- Sign? Digit+                                  ;
                  Expression <- Term (AddOp Term)*                            ;
                  MulOp      <- '*' / '/'                                     ;
                  Term       <- Factor (MulOp Factor)*                        ;
                  AddOp      <- '+'/'-'                                       ;
                  Factor     <- '(' Expression ')' / Number                   ;
              END;


       Using higher-level features of the notation, i.e. the character classes
       (predefined and custom), this example can be rewritten as

              PEG calculator (Expression)
                  Sign       <- [-+] ;
                  Number     <- Sign? <ddigit>+;
                  Expression <- '(' Expression ')' / (Factor (MulOp Factor)*);
                  MulOp      <- [*/];
                  Factor     <- Term (AddOp Term)*;
                  AddOp      <- [-+];
                  Term       <- Number;
              END;


BUGS, IDEAS, FEEDBACK
       This document, and the package it describes, will undoubtedly contain
       bugs and other problems.  Please report such in the category pt of the
       Tcllib SF Trackers [http://sourceforge.net/tracker/?group_id=12883].
       Please also report any ideas for enhancements you may have for either
       package and/or documentation.

KEYWORDS
       EBNF, LL(k), PEG, TDPL, context-free languages, expression, grammar,
       matching, parser, parsing expression, parsing expression grammar, push
       down automaton, recursive descent, state, top-down parsing languages,
       transducer

CATEGORY
       Parsing and Grammars

COPYRIGHT
       Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>




pt                                     1                   pt::peg_language(n)
