Module pegs

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:

notationmeaning
A / ... / ZOrdered 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 ... ZSequence: 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.
$iBack reference to the i``th capture. ``i counts from 1.
$Anchor: Matches at the end of the input. No character is consumed. Same as !..
^Anchor: Matches at the start of the input. No character is consumed.
&EAnd predicate: Indicate success if expression E matches the text ahead; otherwise indicate failure. Do not consume any text.
!ENot predicate: Indicate failure if expression E matches the text ahead; otherwise indicate success. Do not consume any text.
E+One or more: Apply expression E repeatedly to match the text ahead, as long as it succeeds. Consume the matched text (if any) and indicate success if there was at least one match. Otherwise indicate failure.
E*Zero or more: Apply expression E repeatedly to match the text ahead, as long as it succeeds. Consume the matched text (if any). Always indicate success.
E?Zero or one: If expression E matches the text ahead, consume it. Always indicate success.
[s]Character class: If the character ahead appears in the string s, consume it and indicate success. Otherwise indicate failure.
[a-b]Character range: If the character ahead is one from the range a through b, consume it and indicate success. Otherwise indicate failure.
's'String: If the text ahead is the string s, consume it and indicate success. Otherwise indicate failure.
i's'String match ignoring case.
y's'String match ignoring style.
v's'Verbatim string match: Use this to override a global \i or \y modifier.
i$jString match ignoring case for back reference.
y$jString match ignoring style for back reference.
v$jVerbatim 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.
@ESearch: Shorthand for (!E .)* E. (Search loop for the pattern E.)
{@} ECaptured Search: Shorthand for {(!E .)*} E. (Search loop for the pattern E.) Everything until and exluding E is captured.
@@ ESame as {@} E.
A <- ERule: Bind the expression E to the nonterminal symbol A. Left recursive rules are not possible and crash the matching engine.
\identifierBuilt-in macro for a longer expression.
\dddCharacter with decimal code ddd.
\", etcLiteral ", etc.

Built-in macros

macromeaning
\dany decimal digit: [0-9]
\Dany character that is not a decimal digit: [^0-9]
\sany whitespace character: [ \9-\13]
\Sany character that is not a whitespace character: [^ \9-\13]
\wany "word" character: [a-zA-Z0-9_]
\Wany "non-word" character: [^a-zA-Z0-9_]
\asame as [a-zA-Z]
\Asame as [^a-zA-Z]
\nany newline combination: \10 / \13\10 / \13
\iignore case for matching; use this at the start of the PEG
\yignore style for matching; use this at the start of the PEG
\skip patskip 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 ':'.
\identa standard ASCII identifier: [a-zA-Z_][a-zA-Z_0-9]*
\letterany Unicode letter
\upperany Unicode uppercase letter
\lowerany Unicode lowercase letter
\titleany Unicode title letter
\whiteany Unicode whitespace character

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

notationmeaning
\\a single backslash
\*same as '*'
\tnot 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:

  1. Parsing a string into an AST which consists of Peg nodes with the peg proc.
  2. 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.

Types

NonTerminal = ref NonTerminalObj
  Source Edit
Peg = Node
type that represents a PEG   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 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: []
.}
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 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 EIO if an error occurs. This is supposed to be used for quick scripting.   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
proc handleMatches(m: int; n: int; c: openArray[string]): string {.
raises: [], tags: []
.}
  Source Edit

Iterators

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(): expr
expands to charset({'A'..'Z', 'a'..'z'})   Source Edit
template digits(): expr
expands to charset({'0'..'9'})   Source Edit
template whitespace(): expr
expands to charset({' ', '\9'..'\13'})   Source Edit
template identChars(): expr
expands to charset({'a'..'z', 'A'..'Z', '0'..'9', '_'})   Source Edit
template identStartChars(): expr
expands to charset({'A'..'Z', 'a'..'z', '_'})   Source Edit
template ident(): expr
same as [a-zA-Z_][a-zA-z_0-9]*; standard identifier   Source Edit
template natural(): expr
same as \d+   Source Edit
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