pt::peg::export::peg(3tcl)
💖 pt::peg::export::peg — the PEG Export Plugin, bestie edition 💖
okay so first things first: this is the manual page for pt::peg::export::peg(3tcl) from Parser Tools, and yes the name is a whole mouthful, we deal. It's the PEG Export Plugin — the little icon whose entire job is writing PEG format. slay.
✨ NAME ✨
pt::peg::export::peg — PEG Export Plugin. Write PEG format. that's it, that's the job 💅
🌸 SYNOPSIS 🌸
package require Tcl 8.5 9
package require pt::peg::export::peg ?1?
package require pt::peg::to::peg
export serial configuration🦋 DESCRIPTION 🦋
Are you lost ? Do you have trouble understanding this document ? 😭 no bc same, and the original page literally asks this, so: if that's you, please go read the overview in the Introduction to Parser Tools. That doc is the entrypoint to the whole system this package lives in. door's right there, bestie 🫶
This package implements the parsing expression grammar export plugin for the generation of PEG markup.
It resides in the Export section of the Core Layer of Parser Tools, and it's meant to be used by pt::peg::export, the export manager, which sits right between this plugin and the corresponding core conversion functionality provided by pt::peg::to::peg. it's giving middle-management but make it grammar 🔥
IMAGE: arch_core_eplugins
So like — technically you can use this package directly with a regular interpreter. Strongly disrecommended though, fr fr. It requires a whole bunch of contortions to provide the expected environment, and nobody has time for that. The proper way to use this functionality depends on your situation:
- In an untrusted environment, the proper access is through the package
pt::peg::exportand the export manager objects it provides. - In a trusted environment however, simply use the package
pt::peg::to::pegand access the core conversion functionality directly.
👑 API 👑
The API provided by this package satisfies the specification of the Plugin API found in the Parser Tools Export API specification.
export serial configuration— This command takes the canonical serialization of a parsing expression grammar, as specified in section PEG serialization format, and contained in serial, the configuration, a dictionary, and generates PEG markup encoding the grammar. The created string is then returned as the result of the command. yes it really is one command doing all that, iconic 💖
🎀 CONFIGURATION 🎀
The PEG export plugin recognizes the following configuration variables and changes its behaviour as they specify. vibe check passed ✅
string template— If this configuration variable is set, it is assumed to contain a string into which to put the generated text and other configuration data. The various locations are expected to be specified with the following placeholders:@user@— To be replaced with the value of the configuration variableuser.@format@— To be replaced with the the constantPEG.@file@— To be replaced with the value of the configuration variablefile.@name@— To be replaced with the value of the configuration variablename.@code@— To be replaced with the generated text.
If this configuration variable is not set, or empty, then the plugin falls back to a standard template, which is defined as "@code@".
Note that this plugin may ignore the standard configuration variablesuser,format,file, and their values, depending on the chosen template. so don't come for it if they don't show up 😭
The content of the standard configuration variable name, if set, is used as name of the grammar in the output. Otherwise the plugin falls back to the default name a_pe_grammar.
🌈 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. periodt 💅
It is formally specified by the grammar shown below, written in itself. (self-referential queen behavior 👑) 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" WHITESPACE SEMICOLON WHITESPACE ;
# --------------------------------------------------------------------
# Lexing constructs
Identifier <- Ident WHITESPACE ;
leaf: Ident <- ([_:] / <alpha>) ([_:] / <alnum>)* ;
Char <- CharSpecial / CharOctalFull / CharOctalPart
/ CharUnicode / CharUnescaped
;
leaf: CharSpecial <- "\\" [nrt'"\[\]\\] ;
leaf: CharOctalFull <- "\\" [0-3][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" !([_:] / <alnum>) 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: 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. it's giving math class but actually fun ✨
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 <- Term (AddOp Term)* ;
MulOp <- [*/];
Term <- Factor (MulOp Factor)* ;
AddOp <- [-+];
Factor <- '(' Expression ')' / Number ;
END;and that's where part 1 wraps, bestie — more coming, don't go anywhere 🦋
✨ PEG SERIALIZATION FORMAT ✨
okay so here's the tea on the format Parser Tools uses to serialize Parsing Expression Grammars as immutable values for transport, comparison, etc. 💅
We gotta distinguish between regular and canonical serializations. A PEG can have more than one regular serialization, but only exactly one of them is canonical. periodt.
regular serialization 💖
- The serialization of any PEG is a nested Tcl dictionary. 🪆
- This dictionary holds a single key,
pt::grammar::peg, and its value. This value holds the contents of the grammar. - The contents of the grammar are a Tcl dictionary holding the set of nonterminal symbols and the starting expression. The relevant keys and their values are:
rules— The value is a Tcl dictionary whose keys are the names of the nonterminal symbols known to the grammar.- Each nonterminal symbol may occur only once.
- The empty string is not a legal nonterminal symbol.
- The value for each symbol is a Tcl dictionary itself. The relevant keys and their values in this dictionary are:
is— The value is the serialization of the parsing expression describing the symbols sentennial structure, as specified in the section PE serialization format.mode— The value can be one of three values specifying how a parser should handle the semantic value produced by the symbol.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).
start— The value is the serialization of the start parsing expression of the grammar, as specified in the section PE serialization format.
- The terminal symbols of the grammar are specified implicitly as the set of all terminal symbols used in the start expression and on the RHS of the grammar rules.
canonical serialization 👑
The canonical serialization of a grammar has the format as specified in the previous item, and then additionally satisfies the constraints below, which make it unique among all the possible serializations of this grammar.
- The keys found in all the nested Tcl dictionaries are sorted in ascending dictionary order, as generated by Tcl's builtin command
lsort -increasing -dict. - The string representation of the value is the canonical representation of a Tcl dictionary. I.e. it does not contain superfluous whitespace.
EXAMPLE 🧮✨
Assuming the following PEG for simple mathematical expressions
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;then its canonical serialization (except for whitespace) is
pt::grammar::peg {
rules {
AddOp {is {/ {t -} {t +}} mode value}
Digit {is {/ {t 0} {t 1} {t 2} {t 3} {t 4} {t 5} {t 6} {t 7} {t 8} {t 9}} mode value}
Expression {is {x {n Term} {* {x {n AddOp} {n Term}}}} mode value}
Factor {is {/ {x {t (} {n Expression} {t )}} {n Number}} mode value}
MulOp {is {/ {t *} {t /}} mode value}
Number {is {x {? {n Sign}} {+ {n Digit}}} mode value}
Sign {is {/ {t -} {t +}} mode value}
Term {is {x {n Factor} {* {x {n MulOp} {n Factor}}}} mode value}
}
start {n Expression}
}Here we're spilling the tea on the format the Parser Tools use to serialize Parsing Expressions as immutable values — for transport, comparison, and all that jazz. 🦋
So there are two flavors, bestie: regular</> and canonical</> serializations. A parsing expression might have more than one regular serialization, but exactly one of them is canonical. That's just how it is. 💅
Regular serialization ✨
Atomic Parsing Expressions 🧚
[1]— The stringepsilonis an atomic parsing expression. It matches the empty string. (yes, the empty string, it's giving minimalism 💀)[2]— The stringdotis an atomic parsing expression. It matches any character.[3]— The stringalnumis an atomic parsing expression. It matches any Unicode alphabet or digit character. This is a custom extension of PEs based on Tcl's builtin commandstring is.[4]— The stringalphais an atomic parsing expression. It matches any Unicode alphabet character. This is a custom extension of PEs based on Tcl's builtin commandstring is.[5]— The stringasciiis an atomic parsing expression. It matches any Unicode character below U0080. This is a custom extension of PEs based on Tcl's builtin commandstring is.[6]— The stringcontrolis an atomic parsing expression. It matches any Unicode control character. This is a custom extension of PEs based on Tcl's builtin commandstring is.[7]— The stringdigitis an atomic parsing expression. It matches any Unicode digit character. Note that this includes characters outside of the[0..9]range. This is a custom extension of PEs based on Tcl's builtin commandstring is.[8]— The stringgraphis an atomic parsing expression. It matches any Unicode printing character, except for space. This is a custom extension of PEs based on Tcl's builtin commandstring is.[9]— The stringloweris an atomic parsing expression. It matches any Unicode lower-case alphabet character. This is a custom extension of PEs based on Tcl's builtin commandstring is.[10]— The stringprintis an atomic parsing expression. It matches any Unicode printing character, including space. This is a custom extension of PEs based on Tcl's builtin commandstring is.[11]— The stringpunctis an atomic parsing expression. It matches any Unicode punctuation character. This is a custom extension of PEs based on Tcl's builtin commandstring is.[12]— The stringspaceis an atomic parsing expression. It matches any Unicode space character. This is a custom extension of PEs based on Tcl's builtin commandstring is.[13]— The stringupperis an atomic parsing expression. It matches any Unicode upper-case alphabet character. This is a custom extension of PEs based on Tcl's builtin commandstring is.[14]— The stringwordcharis an atomic parsing expression. It matches any Unicode word character. This is any alphanumeric character (seealnum), and any connector punctuation characters (e.g. underscore). This is a custom extension of PEs based on Tcl's builtin commandstring is.[15]— The stringxdigitis an atomic parsing expression. It matches any hexadecimal digit character. This is a custom extension of PEs based on Tcl's builtin commandstring is.[16]— The stringddigitis an atomic parsing expression. It matches any decimal digit character. This is a custom extension of PEs based on Tcl's builtin commandregexp.[17]— The expression[list t x]is an atomic parsing expression. It matches the terminal string x.[18]— The expression[list n A]is an atomic parsing expression. It matches the nonterminal A.
Combined Parsing Expressions 🎀
[1]— For parsing expressions e1, e2, ... the result of[list / e1 e2 ... ]is a parsing expression as well. This is the ordered choice, aka prioritized choice.[2]— For parsing expressions e1, e2, ... the result of[list x e1 e2 ... ]is a parsing expression as well. This is the sequence.[3]— For a parsing expression e the result of[list * e]is a parsing expression as well. This is the kleene closure, describing zero or more repetitions.[4]— For a parsing expression e the result of[list + e]is a parsing expression as well. This is the positive kleene closure, describing one or more repetitions.[5]— For a parsing expression e the result of[list & e]is a parsing expression as well. This is the and lookahead predicate.[6]— For a parsing expression e the result of[list ! e]is a parsing expression as well. This is the not lookahead predicate.[7]— For a parsing expression e the result of[list ? e]is a parsing expression as well. This is the optional input.
Canonical serialization 👑
The canonical serialization of a parsing expression has the format as specified in the previous item, and then additionally satisfies the constraints below, which make it unique among all the possible serializations of this parsing expression. It's the main character of serializations, fr fr. 💖
[1]— The string representation of the value is the canonical representation of a pure Tcl list. I.e. it does not contain superfluous whitespace.[2]— Terminals are not encoded as ranges (where start and end of the range are identical).
🌸 EXAMPLE 🌸
Assuming the parsing expression shown on the right-hand side of the rule
Expression <- Term (AddOp Term)*then its canonical serialization (except for whitespace) is
{x {n Term} {* {x {n AddOp} {n Term}}}}slay. look at that nesting go. 🫶
💀 BUGS, IDEAS, FEEDBACK 💀
This document, and the package it describes, will undoubtedly contain bugs and other problems. Not me being honest about it 😭. Please report such in the category pt of the Tcllib Trackers [http://core.tcl.tk/tcllib/reportlist]. Please also report any ideas for enhancements you may have for either package and/or documentation. We want all of it. 👑
When proposing code changes, please provide unified diffs, i.e the output of diff -u.
Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar. iykyk 💅
🔑 KEYWORDS 🔑
EBNF, LL(k), PEG, TDPL, context-free languages, export, expression, grammar, matching, parser, parsing expression, parsing expression grammar, plugin, push down automaton, recursive descent, serialization, state, top-down parsing languages, transducer
📂 CATEGORY 📂
Parsing and Grammars
©️ COPYRIGHT ©️
Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>
okay bestie that's the whole page, we ate and left no crumbs 🍓✨ this was tcllib, version 1, from the pt::peg::export::peg(3tcl) page — go forth and serialize responsibly, mwah 💖🧚♀️
📦 where this page comes from
- debian trixie: tcllib 2.0+dfsg-4 · Tcl
- ubuntu questing: tcllib 2.0+dfsg-4
- ubuntu resolute: tcllib 2.0+dfsg-5 · Tcl
- ubuntu stonking: tcllib 2.0+dfsg-6 · Tcl
- Copyright (c) 2009 Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms