pt::peg::from::peg(3tcl)
pt::peg::from::peg 💖
so like. this is a page from Parser Tools (the manual section is 3tcl), and it's all about a little package called pt::peg::from::peg. bestie, it reads PEG markup and turns it into a parsing expression grammar. slay.
✨ NAME ✨
pt::peg::from::peg — PEG Conversion. Read PEG format 🎀
✨ SYNOPSIS ✨
package require Tcl 8.5 9
package require pt::peg::from::peg ?1.0.4?
pt::peg::from::peg convert text✨ 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. That document is the entrypoint to the whole system this current package is a part of. no literally, go read it, i'll wait 🫶
This package implements the converter from PEG markup to parsing expression grammars. it's giving translator energy.
It resides in the Import section of the Core Layer of Parser Tools, and can be used either directly with the other packages of this layer, or indirectly through the import manager provided by pt::peg::import. The latter is intented for use in untrusted environments and done through the corresponding import plugin pt::peg::import::peg sitting between converter and import manager. (yes, intented — that's the original spelling, i don't make the rules 💅)
IMAGE: arch_core_iplugins
✨ API ✨
The API provided by this package satisfies the specification of the Converter API found in the Parser Tools Import API specification. no cap, it's fully compliant.
pt::peg::from::peg convert text— okay so this command takes the PEG markup encoding a parsing expression grammar and contained in text, and generates the canonical serialization of said grammar, as specified in section PEG serialization format. The created value is then returned as the result of the command. that's it. that's the whole API. iconic behavior honestly 👑
✨ 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, bestie. 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. (shade. deserved.)
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" 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. a classic. the moment.
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;look at that glow-up ✨ same calculator, way less typing. obsessed.
✨ PEG SERIALIZATION FORMAT ✨
okay bestie so this next bit is literally the format Parser Tools uses to serialize Parsing Expression Grammars as immutable values, for transport, comparison, etc. 🎀
We lowkey distinguish between regular and canonical serializations. A PEG can have more than one regular serialization, but 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. That 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. (not even one time, bestie 🚫)
- 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 symbol's 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. (clean girl energy, no stray spaces ✨)
🌸 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}
}ngl that indentation is giving main character energy, and honestly? it ate. 🫶
✨ PE SERIALIZATION FORMAT ✨
Okay bestie, here's the tea on how the Parser Tools serialize Parsing Expressions as immutable values for transport, comparison, and all that jazz. 💖
We distinguish between regular and canonical serializations. While a parsing expression may have more than one regular serialization, only exactly one of them will be canonical. No cap. 👑
💅 Regular serialization 💅
🌸 Atomic Parsing Expressions 🌸
[1]The stringepsilonis an atomic parsing expression. It matches the empty string. ✨[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 stringx. ⭐[18]The expression[list n A]is an atomic parsing expression. It matches the nonterminalA. 💖
🌈 Combined Parsing Expressions 🌈
[1]For parsing expressionse1,e2, ... the result of[list / e1 e2 ... ]is a parsing expression as well. This is the ordered choice, aka prioritized choice. 💅[2]For parsing expressionse1,e2, ... the result of[list x e1 e2 ... ]is a parsing expression as well. This is the sequence. ✨[3]For a parsing expressionethe result of[list * e]is a parsing expression as well. This is the kleene closure, describing zero or more repetitions. 🔁[4]For a parsing expressionethe 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 expressionethe result of[list & e]is a parsing expression as well. This is the and lookahead predicate. 👀[6]For a parsing expressionethe result of[list ! e]is a parsing expression as well. This is the not lookahead predicate. 🚫[7]For a parsing expressionethe 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.
[1]The string representation of the value is the canonical representation of a pure Tcl list. I.e. it does not contain superfluous whitespace. No extra spaces, we don't do that. 🚫[2]Terminals are not encoded as ranges (where start and end of the range are identical). Keep it simple, bestie. 💅
🌟 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}}}}🐛 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 Trackers [http://core.tcl.tk/tcllib/reportlist]. Please also report any ideas for enhancements you may have for either package and/or documentation. We love feedback, bestie! 💌
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. No cap. 📎
🔑 KEYWORDS 🔑
EBNF, LL(k), PEG, TDPL, context-free languages, conversion, expression, format conversion, grammar, matching, parser, parsing expression, parsing expression grammar, 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>
And that's the whole vibe, bestie! This page is from tcllib version 1.0.4, all about pt::peg::from::peg(3tcl). Stay slay and keep parsing! 💖✨👑
📦 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