Simple PEG (Parsing expression grammar) matching. Uses no memorization, but uses superoperators and symbol inlining to improve performance. Note: Matching performance is hopefully competitive with optimized regular expression engines.

PEG syntax and semantics

A PEG (Parsing expression grammar) is a simple deterministic grammar, that can be directly used for parsing. The current implementation has been designed as a more powerful replacement for regular expressions. UTF-8 is supported.

The notation used for a PEG is similar to that of EBNF:

notation	meaning
`A / ... / Z`	Ordered choice: Apply expressions `A`, ..., `Z`, in this order, to the text ahead, until one of them succeeds and possibly consumes some text. Indicate success if one of expressions succeeded. Otherwise do not consume any text and indicate failure.
`A ... Z`	Sequence: Apply expressions `A`, ..., `Z`, in this order, to consume consecutive portions of the text ahead, as long as they succeed. Indicate success if all succeeded. Otherwise do not consume any text and indicate failure. The sequence's precedence is higher than that of ordered choice: `A B / C` means `(A B) / Z` and not `A (B / Z)`.
`(E)`	Grouping: Parenthesis can be used to change operator priority.
`{E}`	Capture: Apply expression `E` and store the substring that matched `E` into a capture that can be accessed after the matching process.
`$i`	Back reference to the `i`th capture. `i` counts from 1.
`$`	Anchor: Matches at the end of the input. No character is consumed. Same as `!.`.
`^`	Anchor: Matches at the start of the input. No character is consumed.
`&E`	And predicate: Indicate success if expression `E` matches the text ahead; otherwise indicate failure. Do not consume any text.
`!E`	Not predicate: Indicate failure if expression E matches the text ahead; otherwise indicate success. Do not consume any text.
`E+`	One or more: Apply expression `E` repeatedly to match the text ahead, as long as it succeeds. Consume the matched text (if any) and indicate success if there was at least one match. Otherwise indicate failure.
`E*`	Zero or more: Apply expression `E` repeatedly to match the text ahead, as long as it succeeds. Consume the matched text (if any). Always indicate success.
`E?`	Zero or one: If expression `E` matches the text ahead, consume it. Always indicate success.
`[s]`	Character class: If the character ahead appears in the string `s`, consume it and indicate success. Otherwise indicate failure.
`[a-b]`	Character range: If the character ahead is one from the range `a` through `b`, consume it and indicate success. Otherwise indicate failure.
`'s'`	String: If the text ahead is the string `s`, consume it and indicate success. Otherwise indicate failure.
`i's'`	String match ignoring case.
`y's'`	String match ignoring style.
`v's'`	Verbatim string match: Use this to override a global `\i` or `\y` modifier.
`i$j`	String match ignoring case for back reference.
`y$j`	String match ignoring style for back reference.
`v$j`	Verbatim string match for back reference.
`.`	Any character: If there is a character ahead, consume it and indicate success. Otherwise (that is, at the end of input) indicate failure.
`_`	Any Unicode character: If there is an UTF-8 character ahead, consume it and indicate success. Otherwise indicate failure.
`@E`	Search: Shorthand for `(!E .)* E`. (Search loop for the pattern `E`.)
`{@} E`	Captured Search: Shorthand for `{(!E .)*} E`. (Search loop for the pattern `E`.) Everything until and exluding `E` is captured.
`@@ E`	Same as `{@} E`.
`A <- E`	Rule: Bind the expression `E` to the nonterminal symbol `A`. Left recursive rules are not possible and crash the matching engine.
`\identifier`	Built-in macro for a longer expression.
`\ddd`	Character with decimal code ddd.
`\"`, etc	Literal `"`, etc.

Built-in macros

macro	meaning
`\d`	any decimal digit: `[0-9]`
`\D`	any character that is not a decimal digit: `[^0-9]`
`\s`	any whitespace character: `[ \9-\13]`
`\S`	any character that is not a whitespace character: `[^ \9-\13]`
`\w`	any "word" character: `[a-zA-Z0-9_]`
`\W`	any "non-word" character: `[^a-zA-Z0-9_]`
`\a`	same as `[a-zA-Z]`
`\A`	same as `[^a-zA-Z]`
`\n`	any newline combination: `\10 / \13\10 / \13`
`\i`	ignore case for matching; use this at the start of the PEG
`\y`	ignore style for matching; use this at the start of the PEG
`\skip` pat	skip pattern pat before trying to match other tokens; this is useful for whitespace skipping, for example: `\skip(\s*) {\ident} ':' {\ident}` matches key value pairs ignoring whitespace around the `':'`.
`\ident`	a standard ASCII identifier: `[a-zA-Z_][a-zA-Z_0-9]*`
`\letter`	any Unicode letter
`\upper`	any Unicode uppercase letter
`\lower`	any Unicode lowercase letter
`\title`	any Unicode title letter
`\white`	any Unicode whitespace character

A backslash followed by a letter is a built-in macro, otherwise it is used for ordinary escaping:

notation	meaning
`\\`	a single backslash
`\*`	same as `'*'`
`\t`	not a tabulator, but an (unknown) built-in

Supported PEG grammar

The PEG parser implements this grammar (written in PEG syntax):

# Example grammar of PEG in PEG syntax.
# Comments start with '#'.
# First symbol is the start symbol.

grammar <- rule* / expr

identifier <- [A-Za-z][A-Za-z0-9_]*
charsetchar <- "\\" . / [^\]]
charset <- "[" "^"? (charsetchar ("-" charsetchar)?)+ "]"
stringlit <- identifier? ("\"" ("\\" . / [^"])* "\"" /
                          "'" ("\\" . / [^'])* "'")
builtin <- "\\" identifier / [^\13\10]

comment <- '#' @ \n
ig <- (\s / comment)* # things to ignore

rule <- identifier \s* "<-" expr ig
identNoArrow <- identifier !(\s* "<-")
prefixOpr <- ig '&' / ig '!' / ig '@' / ig '{@}' / ig '@@'
literal <- ig identifier? '$' [0-9]+ / '$' / '^' /
           ig identNoArrow /
           ig charset /
           ig stringlit /
           ig builtin /
           ig '.' /
           ig '_' /
           (ig "(" expr ig ")")
postfixOpr <- ig '?' / ig '*' / ig '+'
primary <- prefixOpr* (literal postfixOpr*)

# Concatenation has higher priority than choice:
# ``a b / c`` means ``(a b) / c``

seqExpr <- primary+
expr <- seqExpr (ig "/" expr)*

Note: As a special syntactic extension if the whole PEG is only a single expression, identifiers are not interpreted as non-terminals, but are interpreted as verbatim string:

abc =~ peg"abc" # is true

So it is not necessary to write peg" 'abc' " in the above example.

Examples

Check if s matches Nim's "while" keyword:

s =~ peg" y'while'"

Exchange (key, val)-pairs:

"key: val; key2: val2".replacef(peg"{\ident} \s* ':' \s* {\ident}", "$2: $1")

Determine the #include'ed files of a C file:

for line in lines("myfile.c"):
  if line =~ peg"""s <- ws '#include' ws '"' {[^"]+} '"' ws
                   comment <- '/*' @ '*/' / '//' .*
                   ws <- (comment / \s+)* """:
    echo matches[0]

PEG vs regular expression

As a regular expression \[.*\] matches the longest possible text between '[' and ']'. As a PEG it never matches anything, because a PEG is deterministic: .* consumes the rest of the input, so \] never matches. As a PEG this needs to be written as: \[ ( !\] . )* \] (or \[ @ \]).

Note that the regular expression does not behave as intended either: in the example * should not be greedy, so \[.*?\] should be used instead.

PEG construction

There are two ways to construct a PEG in Nim code:

Parsing a string into an AST which consists of Peg nodes with the peg proc.
Constructing the AST directly with proc calls. This method does not support constructing rules, only simple expressions and is not as convenient. Its only advantage is that it does not pull in the whole PEG parser into your executable.

Imports

strutils, macros, unicode

Types

PegKind = enum pkEmpty, pkAny, pkAnyRune, pkNewLine, pkLetter, pkLower, pkUpper, pkTitle, pkWhitespace, pkTerminal, pkTerminalIgnoreCase, pkTerminalIgnoreStyle, pkChar, pkCharChoice, pkNonTerminal, pkSequence, pkOrderedChoice, pkGreedyRep, pkGreedyRepChar, pkGreedyRepSet, pkGreedyAny, pkOption, pkAndPredicate, pkNotPredicate, pkCapture, pkBackRef, pkBackRefIgnoreCase, pkBackRefIgnoreStyle, pkSearch, pkCapturedSearch, pkRule, pkList, pkStartAnchor: Source Edit
NonTerminalFlag = enum ntDeclared, ntUsed: Source Edit
Peg {...}{.shallow.} = object case kind: PegKind of pkEmpty .. pkWhitespace: nil of pkTerminal, pkTerminalIgnoreCase, pkTerminalIgnoreStyle: term: string of pkChar, pkGreedyRepChar: ch: char of pkCharChoice, pkGreedyRepSet: charChoice: ref set[char] of pkNonTerminal: nt: NonTerminal of pkBackRef .. pkBackRefIgnoreStyle: index: range[0 .. MaxSubpatterns] else: sons: seq[Peg]: type that represents a PEG Source Edit
NonTerminal = ref NonTerminalObj: Source Edit
Captures = object matches: array[0 .. 20 - 1, tuple[first, last: int]] ml: int origStart: int: contains the captured substrings. Source Edit
EInvalidPeg = object of ValueError: raised if an invalid PEG has been detected Source Edit

Consts

MaxSubpatterns = 20: defines the maximum number of subpatterns that can be captured. More subpatterns cannot be captured! Source Edit

Procs

proc kind(p: Peg): PegKind {...}{.raises: [], tags: [].}

Returns the PegKind of a given Peg object. Source Edit

proc term(p: Peg): string {...}{.raises: [], tags: [].}

Returns the string representation of a given Peg variant object where present. Source Edit

proc ch(p: Peg): char {...}{.raises: [], tags: [].}

Returns the char representation of a given Peg variant object where present. Source Edit

proc charChoice(p: Peg): ref set[char] {...}{.raises: [], tags: [].}

Returns the charChoice field of a given Peg variant object where present. Source Edit

proc nt(p: Peg): NonTerminal {...}{.raises: [], tags: [].}

Returns the NonTerminal object of a given Peg variant object where present. Source Edit

proc index(p: Peg): range[0 .. MaxSubpatterns] {...}{.raises: [], tags: [].}

Returns the back-reference index of a captured sub-pattern in the Captures object for a given Peg variant object where present. Source Edit

proc name(nt: NonTerminal): string {...}{.raises: [], tags: [].}

Gets the name of the symbol represented by the parent Peg object variant of a given NonTerminal. Source Edit

proc line(nt: NonTerminal): int {...}{.raises: [], tags: [].}

Gets the line number of the definition of the parent Peg object variant of a given NonTerminal. Source Edit

proc col(nt: NonTerminal): int {...}{.raises: [], tags: [].}

Gets the column number of the definition of the parent Peg object variant of a given NonTerminal. Source Edit

proc flags(nt: NonTerminal): set[NonTerminalFlag] {...}{.raises: [], tags: [].}

Gets the NonTerminalFlag-typed flags field of the parent Peg variant object of a given NonTerminal. Source Edit

proc rule(nt: NonTerminal): Peg {...}{.raises: [], tags: [].}

Gets the Peg object representing the rule definition of the parent Peg object variant of a given NonTerminal. Source Edit

proc term(t: string): Peg {...}{.noSideEffect, gcsafe, extern: "npegs$1Str", raises: [], tags: [].}

constructs a PEG from a terminal string Source Edit

proc termIgnoreCase(t: string): Peg {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

constructs a PEG from a terminal string; ignore case for matching Source Edit

proc termIgnoreStyle(t: string): Peg {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

constructs a PEG from a terminal string; ignore style for matching Source Edit

proc term(t: char): Peg {...}{.noSideEffect, gcsafe, extern: "npegs$1Char", raises: [], tags: [].}

constructs a PEG from a terminal char Source Edit

proc charSet(s: set[char]): Peg {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

constructs a PEG from a character set s Source Edit

proc `/`(a: varargs[Peg]): Peg {...}{.noSideEffect, gcsafe, extern: "npegsOrderedChoice", raises: [], tags: [].}

constructs an ordered choice with the PEGs in a Source Edit

proc sequence(a: varargs[Peg]): Peg {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

constructs a sequence with all the PEGs from a Source Edit

proc `?`(a: Peg): Peg {...}{.noSideEffect, gcsafe, extern: "npegsOptional", raises: [], tags: [].}

constructs an optional for the PEG a Source Edit

proc `*`(a: Peg): Peg {...}{.noSideEffect, gcsafe, extern: "npegsGreedyRep", raises: [], tags: [].}

constructs a "greedy repetition" for the PEG a Source Edit

proc `!*`(a: Peg): Peg {...}{.noSideEffect, gcsafe, extern: "npegsSearch", raises: [], tags: [].}

constructs a "search" for the PEG a Source Edit

proc `!*\`(a: Peg): Peg {...}{.noSideEffect, gcsafe, extern: "npgegsCapturedSearch", raises: [], tags: [].}

constructs a "captured search" for the PEG a Source Edit

proc `+`(a: Peg): Peg {...}{.noSideEffect, gcsafe, extern: "npegsGreedyPosRep", raises: [], tags: [].}

constructs a "greedy positive repetition" with the PEG a Source Edit

proc `&`(a: Peg): Peg {...}{.noSideEffect, gcsafe, extern: "npegsAndPredicate", raises: [], tags: [].}

constructs an "and predicate" with the PEG a Source Edit

proc `!`(a: Peg): Peg {...}{.noSideEffect, gcsafe, extern: "npegsNotPredicate", raises: [], tags: [].}

constructs a "not predicate" with the PEG a Source Edit

proc any(): Peg {...}{.inline, raises: [], tags: [].}

constructs the PEG any character (.) Source Edit

proc anyRune(): Peg {...}{.inline, raises: [], tags: [].}

constructs the PEG any rune (_) Source Edit

proc newLine(): Peg {...}{.inline, raises: [], tags: [].}

constructs the PEG newline (\n) Source Edit

proc unicodeLetter(): Peg {...}{.inline, raises: [], tags: [].}

constructs the PEG \letter which matches any Unicode letter. Source Edit

proc unicodeLower(): Peg {...}{.inline, raises: [], tags: [].}

constructs the PEG \lower which matches any Unicode lowercase letter. Source Edit

proc unicodeUpper(): Peg {...}{.inline, raises: [], tags: [].}

constructs the PEG \upper which matches any Unicode uppercase letter. Source Edit

proc unicodeTitle(): Peg {...}{.inline, raises: [], tags: [].}

constructs the PEG \title which matches any Unicode title letter. Source Edit

proc unicodeWhitespace(): Peg {...}{.inline, raises: [], tags: [].}

constructs the PEG \white which matches any Unicode whitespace character. Source Edit

proc startAnchor(): Peg {...}{.inline, raises: [], tags: [].}

constructs the PEG ^ which matches the start of the input. Source Edit

proc endAnchor(): Peg {...}{.inline, raises: [], tags: [].}

constructs the PEG $ which matches the end of the input. Source Edit

proc capture(a: Peg): Peg {...}{.noSideEffect, gcsafe, extern: "npegsCapture", raises: [], tags: [].}

constructs a capture with the PEG a Source Edit

proc backref(index: range[1 .. MaxSubpatterns]): Peg {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

constructs a back reference of the given index. index starts counting from 1. Source Edit

proc backrefIgnoreCase(index: range[1 .. MaxSubpatterns]): Peg {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

constructs a back reference of the given index. index starts counting from 1. Ignores case for matching. Source Edit

proc backrefIgnoreStyle(index: range[1 .. MaxSubpatterns]): Peg {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

constructs a back reference of the given index. index starts counting from 1. Ignores style for matching. Source Edit

proc nonterminal(n: NonTerminal): Peg {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

constructs a PEG that consists of the nonterminal symbol Source Edit

proc newNonTerminal(name: string; line, column: int): NonTerminal {...}{. noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

constructs a nonterminal symbol Source Edit

proc `$`(r: Peg): string {...}{.noSideEffect, gcsafe, extern: "npegsToString", raises: [], tags: [].}

converts a PEG to its string representation Source Edit

proc bounds(c: Captures; i: range[0 .. 20 - 1]): tuple[first, last: int] {...}{. raises: [], tags: [].}

returns the bounds [first..last] of the i'th capture. Source Edit

proc rawMatch(s: string; p: Peg; start: int; c: var Captures): int {...}{. noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

low-level matching proc that implements the PEG interpreter. Use this for maximum efficiency (every other PEG operation ends up calling this proc). Returns -1 if it does not match, else the length of the match Source Edit

proc matchLen(s: string; pattern: Peg; matches: var openArray[string]; start = 0): int {...}{. noSideEffect, gcsafe, extern: "npegs$1Capture", raises: [], tags: [].}

the same as match, but it returns the length of the match, if there is no match, -1 is returned. Note that a match length of zero can happen. It's possible that a suffix of s remains that does not belong to the match. Source Edit

proc matchLen(s: string; pattern: Peg; start = 0): int {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

proc match(s: string; pattern: Peg; matches: var openArray[string]; start = 0): bool {...}{. noSideEffect, gcsafe, extern: "npegs$1Capture", raises: [], tags: [].}

returns true if s[start..] matches the pattern and the captured substrings in the array matches. If it does not match, nothing is written into matches and false is returned. Source Edit

proc match(s: string; pattern: Peg; start = 0): bool {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

returns true if s matches the pattern beginning from start. Source Edit

proc find(s: string; pattern: Peg; matches: var openArray[string]; start = 0): int {...}{. noSideEffect, gcsafe, extern: "npegs$1Capture", raises: [], tags: [].}

returns the starting position of pattern in s and the captured substrings in the array matches. If it does not match, nothing is written into matches and -1 is returned. Source Edit

proc findBounds(s: string; pattern: Peg; matches: var openArray[string]; start = 0): tuple[first, last: int] {...}{.noSideEffect, gcsafe, extern: "npegs$1Capture", raises: [], tags: [].}

returns the starting position and end position of pattern in s and the captured substrings in the array matches. If it does not match, nothing is written into matches and (-1,0) is returned. Source Edit

proc find(s: string; pattern: Peg; start = 0): int {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

returns the starting position of pattern in s. If it does not match, -1 is returned. Source Edit

proc findAll(s: string; pattern: Peg; start = 0): seq[string] {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

returns all matching substrings of s that match pattern. If it does not match, @[] is returned. Source Edit

proc contains(s: string; pattern: Peg; start = 0): bool {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

same as find(s, pattern, start) >= 0 Source Edit

proc contains(s: string; pattern: Peg; matches: var openArray[string]; start = 0): bool {...}{. noSideEffect, gcsafe, extern: "npegs$1Capture", raises: [], tags: [].}

same as find(s, pattern, matches, start) >= 0 Source Edit

proc startsWith(s: string; prefix: Peg; start = 0): bool {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

returns true if s starts with the pattern prefix Source Edit

proc endsWith(s: string; suffix: Peg; start = 0): bool {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

returns true if s ends with the pattern suffix Source Edit

proc replacef(s: string; sub: Peg; by: string): string {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [ValueError], tags: [].}

Replaces sub in s by the string by. Captures can be accessed in by with the notation $i and $# (see strutils.`%`). Examples:

"var1=key; var2=key2".replacef(peg"{\ident}'='{\ident}", "$1<-$2$2")

Results in:

"var1<-keykey; val2<-key2key2"

Source Edit

proc replace(s: string; sub: Peg; by = ""): string {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

Replaces sub in s by the string by. Captures cannot be accessed in by. Source Edit

proc parallelReplace(s: string; subs: varargs[tuple[pattern: Peg, repl: string]]): string {...}{. noSideEffect, gcsafe, extern: "npegs$1", raises: [ValueError], tags: [].}

Returns a modified copy of s with the substitutions in subs applied in parallel. Source Edit

proc replace(s: string; sub: Peg; cb: proc (match: int; cnt: int; caps: openArray[string]): string): string {...}{. gcsafe, extern: "npegs$1cb", raises: [], tags: [].}

Replaces sub in s by the resulting strings from the callback. The callback proc receives the index of the current match (starting with 0), the count of captures and an open array with the captures of each match. Examples:

proc handleMatches*(m: int, n: int, c: openArray[string]): string =
  result = ""
  if m > 0:
    result.add ", "
  result.add case n:
    of 2: c[0].toLower & ": '" & c[1] & "'"
    of 1: c[0].toLower & ": ''"
    else: ""

let s = "Var1=key1;var2=Key2;   VAR3"
echo s.replace(peg"{\ident}('='{\ident})* ';'* \s*", handleMatches)

Results in:

"var1: 'key1', var2: 'Key2', var3: ''"

Source Edit

proc transformFile(infile, outfile: string; subs: varargs[tuple[pattern: Peg, repl: string]]) {...}{.gcsafe, extern: "npegs$1", raises: [IOError, ValueError], tags: [ReadIOEffect, WriteIOEffect].}

reads in the file infile, performs a parallel replacement (calls parallelReplace) and writes back to outfile. Raises IOError if an error occurs. This is supposed to be used for quick scripting.

Note: this proc does not exist while using the JS backend.

Source Edit

proc split(s: string; sep: Peg): seq[string] {...}{.noSideEffect, gcsafe, extern: "npegs$1", raises: [], tags: [].}

Splits the string s into substrings. Source Edit

proc parsePeg(pattern: string; filename = "pattern"; line = 1; col = 0): Peg {...}{. raises: [ValueError, EInvalidPeg, Exception], tags: [RootEffect].}

constructs a Peg object from pattern. filename, line, col are used for error messages, but they only provide start offsets. parsePeg keeps track of line and column numbers within pattern. Source Edit

proc peg(pattern: string): Peg {...}{.raises: [ValueError, EInvalidPeg, Exception], tags: [RootEffect].}

constructs a Peg object from the pattern. The short name has been chosen to encourage its use as a raw string modifier:

peg"{\ident} \s* '=' \s* {.*}"

Source Edit

proc escapePeg(s: string): string {...}{.raises: [], tags: [].}

escapes s so that it is matched verbatim when used as a peg. Source Edit

Iterators

iterator items(p: Peg): Peg {...}{.inline, raises: [], tags: [].}

Yields the child nodes of a Peg variant object where present. Source Edit

iterator pairs(p: Peg): (int, Peg) {...}{.inline, raises: [], tags: [].}

Yields the indices and child nodes of a Peg variant object where present. Source Edit

iterator findAll(s: string; pattern: Peg; start = 0): string {...}{.raises: [], tags: [].}

yields all matching substrings of s that match pattern. Source Edit

iterator split(s: string; sep: Peg): string {...}{.raises: [], tags: [].}

Splits the string s into substrings.

Substrings are separated by the PEG sep. Examples:

for word in split("00232this02939is39an22example111", peg"\d+"):
  writeLine(stdout, word)

Results in:

"this"
"is"
"an"
"example"

Source Edit

Templates

template letters(): Peg

expands to charset({'A'..'Z', 'a'..'z'}) Source Edit

template digits(): Peg

expands to charset({'0'..'9'}) Source Edit

template whitespace(): Peg

expands to charset({' ', '\9'..'\13'}) Source Edit

template identChars(): Peg

expands to charset({'a'..'z', 'A'..'Z', '0'..'9', '_'}) Source Edit

template identStartChars(): Peg

expands to charset({'A'..'Z', 'a'..'z', '_'}) Source Edit

template ident(): Peg

same as [a-zA-Z_][a-zA-z_0-9]*; standard identifier Source Edit

template natural(): Peg

same as \d+ Source Edit

template eventParser(pegAst, handlers: untyped): (proc (s: string): int)

Generates an interpreting event parser proc according to the specified PEG AST and handler code blocks. The proc can be called with a string to be parsed and will execute the handler code blocks whenever their associated grammar element is matched. It returns -1 if the string does not match, else the length of the total match. The following example code evaluates an arithmetic expression defined by a simple PEG:

import strutils, pegs

let
  pegAst = """
Expr    <- Sum
Sum     <- Product (('+' / '-')Product)*
Product <- Value (('*' / '/')Value)*
Value   <- [0-9]+ / '(' Expr ')'
  """.peg
  txt = "(5+3)/2-7*22"

var
  pStack: seq[string] = @[]
  valStack: seq[float] = @[]
  opStack = ""
let
  parseArithExpr = pegAst.eventParser:
    pkNonTerminal:
      enter:
        pStack.add p.nt.name
      leave:
        pStack.setLen pStack.high
        if length > 0:
          let matchStr = s.substr(start, start+length-1)
          case p.nt.name
          of "Value":
            try:
              valStack.add matchStr.parseFloat
              echo valStack
            except ValueError:
              discard
          of "Sum", "Product":
            try:
              let val = matchStr.parseFloat
            except ValueError:
              if valStack.len > 1 and opStack.len > 0:
                valStack[^2] = case opStack[^1]
                of '+': valStack[^2] + valStack[^1]
                of '-': valStack[^2] - valStack[^1]
                of '*': valStack[^2] * valStack[^1]
                else: valStack[^2] / valStack[^1]
                valStack.setLen valStack.high
                echo valStack
                opStack.setLen opStack.high
                echo opStack
    pkChar:
      leave:
        if length == 1 and "Value" != pStack[^1]:
          let matchChar = s[start]
          opStack.add matchChar
          echo opStack

let pLen = parseArithExpr(txt)

The handlers parameter consists of code blocks for PegKinds, which define the grammar elements of interest. Each block can contain handler code to be executed when the parser enters and leaves text matching the grammar element. An enter handler can access the specific PEG AST node being matched as p, the entire parsed string as s and the position of the matched text segment in s as start. A leave handler can access p, s, start and also the length of the matched text segment as length. For an unsuccessful match, the enter and leave handlers will be executed, with length set to -1.

Symbols declared in an enter handler can be made visible in the corresponding leave handler by annotating them with an inject pragma.

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template `=~`(s: string; pattern: Peg): bool

This calls match with an implicit declared matches array that can be used in the scope of the =~ call:

if line =~ peg"\s* {\w+} \s* '=' \s* {\w+}":
  # matches a key=value pair:
  echo("Key: ", matches[0])
  echo("Value: ", matches[1])
elif line =~ peg"\s*{'#'.*}":
  # matches a comment
  # note that the implicit ``matches`` array is different from the
  # ``matches`` array of the first branch
  echo("comment: ", matches[0])
else:
  echo("syntax error")

Source Edit