This is a pure Nim port of the pattern matching code from Lua 5.3, and still a
work in progress.



The chapter 'patterns' from the Lua manual:

  Patterns are described by regular strings, which are interpreted as
  patterns by the pattern-matching functions string.find, string.gmatch,
  string.gsub, and string.match. This section describes the syntax and the
  meaning (that is, what they match) of these strings.

Character Class:

  A character class is used to represent a set of characters. The following
  combinations are allowed in describing a character class:

    -  x: (where x is not one of the magic characters ^$()%.[]*+-?) represents the
          character x itself.
    -  .: (a dot) represents all characters.
    - %a: represents all letters.
    - %c: represents all control characters.
    - %d: represents all digits.
    - %g: represents all printable characters except space.
    - %l: represents all lowercase letters.
    - %p: represents all punctuation characters.
    - %s: represents all space characters.
    - %u: represents all uppercase letters.
    - %w: represents all alphanumeric characters.
    - %x: represents all hexadecimal digits.
    - %x: (where x is any non-alphanumeric character) represents the character x.
	  This is the standard way to escape the magic characters. Any
	  non-alphanumeric character (including all punctuation characters,
	  even the non-magical) can be preceded by a '%' when used to represent
	  itself in a pattern.
    - [set]: represents the class which is the union of all characters in set.
	  A range of characters can be specified by separating the end
	  characters of the range, in ascending order, with a '-'. All classes
	  %x described above can also be used as components in set. All other
	  characters in set represent themselves. For example, [%w_] (or [_%w])
	  represents all alphanumeric characters plus the underscore, [0-7]
	  represents the octal digits, and [0-7%l%-] represents the octal
	  digits plus the lowercase letters plus the '-' character.

    You can put a closing square bracket in a set by positioning it as the
    first character in the set. You can put an hyphen in a set by positioning
    it as the first or the last character in the set. (You can also use an
    escape for both cases.)

    The interaction between ranges and classes is not defined. Therefore,
    patterns like [%a-z] or [a-%%] have no meaning.  [^set]: represents the
    complement of set, where set is interpreted as above.

  For all classes represented by single letters (%a, %c, etc.), the corresponding
  uppercase letter represents the complement of the class. For instance, %S
  represents all non-space characters.
  
  The definitions of letter, space, and other character groups depend on the
  current locale. In particular, the class [a-z] may not be equivalent to %l.

Pattern Item:

  A pattern item can be

    - a single character class, which matches any single character in the class;
    - a single character class followed by '*', which matches zero or more
      repetitions of characters in the class. These repetition items will
      always match the longest possible sequence;
    - a single character class followed by '+', which matches one or more
      repetitions of characters in the class. These repetition items will
      always match the longest possible sequence;
    - a single character class followed by '-', which also matches zero or more
      repetitions of characters in the class. Unlike '*', these repetition
      items will always match the shortest possible sequence;
    - a single character class followed by '?', which matches zero or one
      occurrence of a character in the class. It always matches one occurrence
      if possible;
    - %n, for n between 1 and 9; such item matches a substring equal to the
      n-th captured string (see below);
    - %bxy, where x and y are two distinct characters; such item matches
      strings that start with x, end with y, and where the x and y are
      balanced. This means that, if one reads the string from left to right,
      counting +1 for an x and -1 for a y, the ending y is the first y where
      the count reaches 0. For instance, the item %b() matches expressions with
      balanced parentheses.
    - %f[set], a frontier pattern; such item matches an empty string at any
      position such that the next character belongs to set and the previous
      character does not belong to set. The set set is interpreted as
      previously described. The beginning and the end of the subject are
      handled as if they were the character '\0'.

Pattern:

  A pattern is a sequence of pattern items. A caret '^' at the beginning of a
  pattern anchors the match at the beginning of the subject string. A '$' at the
  end of a pattern anchors the match at the end of the subject string. At other
  positions, '^' and '$' have no special meaning and represent themselves.

Captures:

  A pattern can contain sub-patterns enclosed in parentheses; they describe
  captures. When a match succeeds, the substrings of the subject string that
  match captures are stored (captured) for future use. Captures are numbered
  according to their left parentheses. For instance, in the pattern
  "(a*(.)%w(%s*))", the part of the string matching "a*(.)%w(%s*)" is stored as
  the first capture (and therefore has number 1); the character matching "." is
  captured with number 2, and the part matching "%s*" has number 3.
  
  As a special case, the empty capture () captures the current string position (a
  number). For instance, if we apply the pattern "()aa()" on the string "flaaap",
  there will be two captures: 3 and 5.