Actually several packages:

   * grammar::peg - Construction and manipulation of parsing expression grammars
   * grammar::peg::interp - Interpreter for parsing expression grammars.

Documentation:
   * http://tcllib.sourceforge.net/doc/peg.html
   * http://tcllib.sourceforge.net/doc/peg_interp.html

----
3-9-8: I hope the following may help others who are trying to use the "grammar::peg" package.
These were tested under "etcl" (Evolane Tcl/Tk Engine Version 1.0-rc26  See http://wiki.tcl.tk/15260 ).
======
1% # For now, just load the 'grammar::peg^ package
2% package require grammar::peg
0.1

3% #initialize a new (abeit empty) grammar.  The start symbol will be, by default, 'epsilon^.
4% grammar::peg myGrammar
::myGrammar

5% # add a nonterminal to it
6% ::myGrammar nonterminal add myDigit {/ {t 0} {t 1} {t 2}}

7% # show its serialization
8% ::myGrammar serialize
grammar::pegc {myDigit {/ {t 0} {t 1} {t 2}}} {myDigit value} epsilon

9% # change the start symbol to the nonterminal 'myDigit^
10% ::myGrammar start {/ {n myDigit}}

11% # show its serialization
12% ::myGrammar serialize
grammar::pegc {myDigit {/ {t 0} {t 1} {t 2}}} {myDigit value} {/ {n myDigit}}

13% # Now we show how you could, instead, have started with a list representing
14% # the grammar. Since "myGrammar" exists, we will make "myGrammarFromList".
15% package require struct::list
1.6.1

16% # (Incidentally, Andreas Kupries states: 'struct::list' is a package in Tcllib, like grammar::peg, and
17% #                  guesses that grammar::peg forgot to 'package require' it.)
18% set mySerialization [list grammar::pegc \
> {myDigit {/ {t 0} {t 1} {t 2}}} {myDigit value} {/ {n myDigit}} \
> ]
grammar::pegc {myDigit {/ {t 0} {t 1} {t 2}}} {myDigit value} {/ {n myDigit}}

19% grammar::peg myGrammarFromList deserialize $mySerialization
::myGrammarFromList

20% # Next we attempt to use (interpret) the grammar on an input string.
21% #Load all relevant packages (this may be overkill)
22% set pattern {grammar::me|grammar::peg}; foreach packageName [package names] {if [regexp $pattern $packageName] {puts "[catch {package require $packageName}]  $packageName"}}
0  grammar::peg::interp
0  grammar::peg
0  grammar::me::tcl
0  grammar::me::cpu
0  grammar::me::util
0  grammar::me::cpu::core
0  grammar::me::cpu::gasm

23% # Initialize the interpreter.
24% grammar::peg::interp::setup ::myGrammar
25% # As of 3-9-8, that's as far as I have gotten.  The documentation at http://tcllib.sourceforge.net/doc/peg_interp.html
26% # suggests using 
27% #                            ::grammar::peg::interp::parse nextcmd errorvar astvar 
28% #" which "interprets the loaded grammar and tries to match it against the stream of characters represented by the command prefix nextcmd".
29% # so I put the string "102" in "myFile.txt" and tried the following, but, as you can see, was unsuccessful.

30% set f [open c:/myFile.txt r+]
file37bf6c8
31% chan configure $f -encoding cp1252
32% set offset 0
0
33% ::grammar::peg::interp::parse $f myErrorVar myAstVar
invalid command name "file37bf6c8"
34% chan close $f
======
----------

[JBR] I've made my own attempt to create a grammar::peg example.  Maybe someone who knows can make this example work. Thanks.

 #!/usr/bin/env tclsh8.6
 #

 package require grammar::peg
 package require grammar::peg::interp

 proc parse-string { string } {
    coroutine next-char apply { { string } {
        yield

        set i 1
        foreach ch [split $string {}] {
            yield [list $ch 1 1 $i]

            incr i
        }

        return {} } } $string
 }

 ::grammar::peg parser

 parser nonterminal add Digit { / { t 1 } { t 2 } { t 3 } { t 4 } { t 5 } }
 parser nonterminal add Int   { + { n Digit } }
 parser start { n Int }

 parse-string 54321

 grammar::peg::interp::setup parser
 grammar::peg::interp::parse next-char err ast


When run I get this:

 wrong # args: should be "ict_match_token tok msg"
    while executing
 "ict_match_token "Expected $ch""
    (procedure "MatchExpr" line 40)
    invoked from within
 "MatchExpr $e"
    (procedure "MatchExpr" line 236)
    invoked from within
 "MatchExpr $ru($nt)"
    (procedure "MatchExpr" line 75)
    invoked from within
 "MatchExpr $sub"
    (procedure "MatchExpr" line 159)
    invoked from within
 "MatchExpr $ru($nt)"
    (procedure "MatchExpr" line 75)
    invoked from within
 "MatchExpr $se"
    (procedure "grammar::peg::interp::parse" line 9)
    invoked from within
 "grammar::peg::interp::parse next-char err ast"
    (file "./ISBL" line 30)


----

%| [Category Package], a part of [Tcllib] | [Category Parsing] |%