pt::peg::from::peg(n)
✨ NAME ✨
pt::peg::from::peg — PEG Conversion. Read PEG format.
okay bestie, this is the manual page for pt::peg::from::peg(n) from Parser Tools. it's giving converter-from-PEG-markup-to-parsing-expression-grammars, and it's lowkey iconic. 🫶
🦋 SYNOPSIS 🦋
package require Tcl 8.5 9
package require pt::peg::from::peg ?1.0.4?
pt::peg::from::peg convert text💖 DESCRIPTION 💖
okay so if you're lost or this document is giving cryptic, please go read the overview in the Introduction to Parser Tools. that doc is the entrypoint to the whole system this package is part of. fr fr.
this package implements the converter from PEG markup to parsing expression grammars. no cap.
it lives in the Import section of the Core Layer of Parser Tools, and you can use it either directly with the other packages of this layer, or indirectly through the import manager from pt::peg::import. the indirect route is intented for use in untrusted environments, and it goes through the corresponding import plugin pt::peg::import::peg sitting between converter and import manager.
IMAGE: arch_core_iplugins
🎀 API 🎀
the API this package provides satisfies the spec of the Converter API found in the Parser Tools Import API specification.
pt::peg::from::peg convert text
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. slay. 💅
🌸 PEG SPECIFICATION LANGUAGE 🌸
peg, a language for the specification of parsing expression grammars, is meant to be human readable and writable, yet strict enough to allow machine processing. like any computer language. it was defined to make writing the specification of a grammar easy, something the other formats found in Parser Tools do not lend themselves to. periodt.
it is formally specified by the grammar shown below, written in itself. for a tutorial / introduction to the language, please go 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.
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;🎀 PEG SERIALIZATION FORMAT 🎀
okay so bestie, this is the part where we talk about how the Parser Tools write a Parsing Expression Grammar out as an immutable value — like, for transport, for comparison, all of that. 💖
So there are two flavors: regular serialization and canonical serialization. A PEG can have more than one regular serialization, but exactly one of them is canonical. no cap. 💅
✨ 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 symbol's sentential 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 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 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. it's giving "one true form". ✨
- 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}
}bestie, notice how every single rule is in there, and how themodefor all of them isvaluein this example. don't go thinking that's the only option —leafandvoidare real too, we just covered them above. 🫶
💖 PE SERIALIZATION FORMAT 💖
okay so bestie, here we're specifying the format used by the Parser Tools to serialize Parsing Expressions as immutable values for transport, comparison, etc. 💅
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 ✨
- The string
epsilonis an atomic parsing expression. It matches the empty string. literally nothing. 💀 - The string
dotis an atomic parsing expression. It matches any character. any single character, periodt. 👑 - The string
alnumis 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. 🍓 - The string
alphais an atomic parsing expression. It matches any Unicode alphabet character. This is a custom extension of PEs based on Tcl's builtin commandstring is. 🌸 - The string
asciiis 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. 🦋 - The string
controlis an atomic parsing expression. It matches any Unicode control character. This is a custom extension of PEs based on Tcl's builtin commandstring is. 💀 - The string
digitis 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. 🔥 - The string
graphis 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. 👑 - The string
loweris 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. 💜 - The string
printis 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. 💖 - The string
punctis an atomic parsing expression. It matches any Unicode punctuation character. This is a custom extension of PEs based on Tcl's builtin commandstring is. 💅 - The string
spaceis an atomic parsing expression. It matches any Unicode space character. This is a custom extension of PEs based on Tcl's builtin commandstring is. 🌸 - The string
upperis 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. 👑 - The string
wordcharis 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. 🧚 - The string
xdigitis an atomic parsing expression. It matches any hexadecimal digit character. This is a custom extension of PEs based on Tcl's builtin commandstring is. 🎀 - The string
ddigitis an atomic parsing expression. It matches any decimal digit character. This is a custom extension of PEs based on Tcl's builtin commandregexp. 🎀 - The expression
[list t x]is an atomic parsing expression. It matches the terminal string x. 🔥 - The expression
[list n A]is an atomic parsing expression. It matches the nonterminal A. 💜
💅 Combined Parsing Expressions 💅
- 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. slay. 💅 - For parsing expressions e1, e2, ... the result of
[list x e1 e2 ... ]is a parsing expression as well. This is the sequence. periodt. ✨ - 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. we love a flexible queen. 👑 - 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. one or more, no cap. 🔥 - For a parsing expression e the result of
[list & e]is a parsing expression as well. This is the and lookahead predicate. ✨ - For a parsing expression e the result of
[list ! e]is a parsing expression as well. This is the not lookahead predicate. not me looking ahead 🙈 - For a parsing expression e the result of
[list ? e]is a parsing expression as well. This is the optional input. maybe she's optional, maybe she's not 💅
🦋 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 giving ✨main character✨ energy.
- 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, bestie. 🧚
- Terminals are not encoded as ranges (where start and end of the range are identical). periodt. 💖
🌈 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're all ears 🫶
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.
🔑 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>
okay bestie, that's the whole vibe. this page came from source tcllib, version 1.0.4, and it's pt::peg::from::peg(n). mwah 💖✨
📦 where this page comes from
- fedora 43: tcllib 2.0-3.fc43 · TCL
- fedora 44: tcllib 2.0-4.fc44 · TCL
- nixos 26.05: tcllib 2.0
- nixpkgs unstable: tcllib 2.0
- 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