pt::peg::import::peg(n)
✨ pt::peg::import::peg ✨
okay bestie, welcome to pt::peg::import::peg(n) — the PEG Import Plugin, aka the little icon that reads PEG format for you. it lives in Parser Tools and it is genuinely so good. let's get into it 💖
🎀 NAME 🎀
pt::peg::import::peg — PEG Import Plugin. Read PEG format ✨
🦋 SYNOPSIS 🦋
package require Tcl 8.5 9
package require pt::peg::import::peg ?1.1?
package require pt::peg::to::peg
import text💅 DESCRIPTION 💅
Are you lost? 😭 Do you have trouble understanding this document? fr fr if that's you, please go read the overview in the Introduction to Parser Tools. That doc is literally the entrypoint to the whole system this package is part of. no cap. 🫶
So: this package implements the parsing expression grammar import plugin that processes PEG markup. 🎀
It resides in the Import section of the Core Layer of Parser Tools, and it's intended to be used by pt::peg::import, the import manager — it sits right between the manager and the corresponding core conversion functionality provided by pt::peg::from::peg. it's giving middle child but make it essential. 👑
IMAGE: arch_core_iplugins
okay so technically you can use this package directly with a regular interpreter, but that is strongly disrecommended and requires a number of contortions to provide the expected environment. 💀 the proper way to use this functionality depends on the situation:
- In an untrusted environment the proper access is through the package
pt::peg::importand the import manager objects it provides. - In a trusted environment however, simply use the package
pt::peg::from::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 Import API specification. ✨
import 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 the section PEG serialization format. The created value is then returned as the result of the command. periodt. 💅
🧚 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. 🫶
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. 🔥
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;okay bestie that's part 1 — hold on, there's more coming, don't go anywhere 💖
✨ PEG SERIALIZATION FORMAT ✨
Okay bestie, this is the part where we talk about how Parser Tools turns a Parsing Expression Grammar into something you can ship around, compare, etc. It's giving immutable values for transport, comparison, and all that. 💖
We got two flavors: regular and canonical. 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. (Yes, sentennial. We keep it exactly, no edits, iykyk. 💅)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. No cap. 💯
- 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}
}💖 PE SERIALIZATION FORMAT 💖
okay bestie, so here's the tea on how the Parser Tools serialize Parsing Expressions into immutable values — you know, for shipping them around, comparing them, that whole vibe. 🫶
We got two flavors: regular and canonical serializations. A parsing expression can have a whole bunch of regular serializations, but only exactly one of them gets to be the canonical one. periodt. 💅
🌸 Regular serialization 🌸
🦋 Atomic Parsing Expressions 🦋
- The string
epsilonis an atomic parsing expression, bestie. It matches the empty string. ✨ - The string
dotis an atomic parsing expression. It matches any character. 💫 - 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. iykyk 😌 - 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 stringx. 🎯 - The expression
[list n A]is an atomic parsing expression. It matches the nonterminalA. 🏷️
🧚 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. 🤔 - For parsing expressions
e1,e2, ... the result of[list x e1 e2 ... ]is a parsing expression as well. This is the sequence. 🔗 - For a parsing expression
ethe result of[list * e]is a parsing expression as well. This is the kleene closure, describing zero or more repetitions. ♾️ - For a parsing expression
ethe result of[list + e]is a parsing expression as well. This is the positive kleene closure, describing one or more repetitions. ➕ - For a parsing expression
ethe result of[list & e]is a parsing expression as well. This is the and lookahead predicate. 👀 - For a parsing expression
ethe result of[list ! e]is a parsing expression as well. This is the not lookahead predicate. 🙅 - For a parsing expression
ethe 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. 💎
- The string representation of the value is the canonical representation of a pure Tcl list. I.e. it does not contain superfluous whitespace. 🧹
- 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}}}}😭 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. 🙏
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, expression, grammar, import, 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>
This masterpiece was brought to you by tcllib, version 1.1, and it's the pt::peg::import::peg(n) manpage. 💖 Stay slaying, bestie! ✨🦋🌸
📦 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