Wow, I didn’t realize I needed this library. Much, much easier to use than regular expressions. PEG seems like it would be an excellent addition to the core library as an alternative to Regex.

The following code is me just messing around trying to get things to work, but I hope it helps someone.

defmodule PegasusExample do
  import NimbleParsec
  require Pegasus
  @moduledoc """

  Examples for PEG parsing.

  PegasusExample.get_pairs("grass=4,horse=1, star=2")

  From https://github.com/xored/peg/blob/master/docs/grammar-examples.md
  PegasusExample.get_timestamp("2009-09-22T06:59:28")
  PegasusExample.get_timestamp("2009-09-22 06:59:28")
  PegasusExample.get_timestamp("Fri Jun 17 03:50:56 PDT 2011")
  PegasusExample.get_timestamp("2010-10-26 10:00:53.360")
  """

  @parser_options [
    Pair: [tag: :pair],
    Word: [tag: :word],
    Number: [tag: :number],
    Space: [ignore: true],
    Separator: [ignore: true],
    Equals: [ignore: true],
  ]
  Pegasus.parser_from_string(
  """
  List <- Pair (Space* Separator Space* Pair)*
  Pair <- Word Equals Number
  Word <- [A-Za-z0-9_]+
  Number <- [0-9]+
  Space           <- ' ' / '\t' / EndOfLine
  EndOfLine       <- '\r\n' / '\n' / '\r'
  EndOfFile       <- !.
  Separator       <- ','
  Equals          <- '='
  """,
  @parser_options
  )
  defparsec :get_pairs, parsec(:List)

  @parser_timestamp [
    Hour: [tag: :hour],
    Minute: [tag: :minute],
    Second: [tag: :second],
    Year: [tag: :year],
    Month: [tag: :month],
    Day: [tag: :day],
    TZ: [tag: :tz],
    Space: [ignore: true],
    Separator: [ignore: true],
    Equals: [ignore: true],
  ]
  Pegasus.parser_from_string(
  """
    Timestamp <- DateTime / FreeDateTime

    # Times
    Hour <- [0-1] [0-9] / '2' [0-4]
    Minute <- [0-5] [0-9]
    Second <- [0-5] [0-9] / '60'
    Fraction <- ('.' / ',') [0-9]+
    IsoTz <- 'Z' / ('+' / '-') Hour (':'? Minute)?
    TzL <- [A-Z]
    TzAbbr <- TzL TzL (TzL (TzL TzL?)?)?
    TZ <- IsoTz / TzAbbr
    HM <- Hour ':' Minute Fraction?
    HMS <- Hour ':' Minute ':' Second Fraction?
    Time <- ('T' ' '?)? (HMS / HM) (' '? TZ)?

    # Dates
    Year <- [0-9] [0-9] [0-9] [0-9]
    Month <- '0' [1-9] / '1' [0-2]
    Day <- '0' [1-9] / [1-2] [0-9] / '3' [0-1]
    Date <- Year '-' Month ('-' Day)?

    # Combined
    DateTime <- Date ' '? Time

    # Free style
    MonthAbbr <- 'Jan' / 'Feb' / 'Mar' / 'Apr' / 'May' / 'Jun' / 'Jul' / 'Aug' / 'Sep' / 'Sept' / 'Oct' / 'Nov' / 'Dec'
    WeekDayAbbr <- 'Mon' / 'Tu' / 'Tue' / 'Tues' / 'Wed' / 'Th' / 'Thu' / 'Thur' / 'Thurs' / 'Fri' / 'Sat' / 'Sun'
    FreeDateTime <- WeekDayAbbr ' ' MonthAbbr ' ' Day ' ' Time ' ' Year
 """,
   @parser_timestamp)
   defparsec :get_timestamp, parsec(:Timestamp)
end

iex>  PegasusExample.get_pairs("grass=4,horse=1, star=2")
{:ok,
 [
   pair: [word: ~c"grass", number: ~c"4"],
   pair: [word: ~c"horse", number: ~c"1"],
   pair: [word: ~c"star", number: ~c"2"]
 ], "", %{}, {1, 0}, 23}

Wow. Combining PEG grammars with Nimbleparsec is :chef_kiss:

Most of these:

And this:

https://github.com/E-xyza/zig_parser/blob/main/lib/grammar/grammar.y