regex.cr

require "./regex/*"

# A `Regex` represents a regular expression, a pattern that describes the
# contents of strings. A `Regex` can determine whether or not a string matches
# its description, and extract the parts of the string that match.
#
# A `Regex` can be created using the literal syntax, in which it is delimited by
# forward slashes (`/`):
#
# ```
# /hay/ =~ "haystack"   # => 0
# /y/.match("haystack") # => #<Regex::MatchData "y">
# ```
#
# Interpolation works in regular expression literals just as it does in string
# literals. Be aware that using this feature will cause an exception to be
# raised at runtime, if the resulting string would not be a valid regular
# expression.
#
# ```
# x = "a"
# /#{x}/.match("asdf") # => #<Regex::MatchData "a">
# x = "("
# /#{x}/ # raises ArgumentError
# ```
#
# When we check to see if a particular regular expression describes a string,
# we can say that we are performing a match or matching one against the other.
# If we find that a regular expression does describe a string, we say that it
# matches, and we can refer to a part of the string that was described as
# a match.
#
# Here `"haystack"` does not contain the pattern `/needle/`, so it doesn't match:
#
# ```
# /needle/.match("haystack") # => nil
# ```
#
# Here `"haystack"` contains the pattern `/hay/`, so it matches:
#
# ```
# /hay/.match("haystack") # => #<Regex::MatchData "hay">
# ```
#
# Regex methods that perform a match usually return a truthy value if there was
# a match and `nil` if there was no match. After performing a match, the
# special variable `$~` will be an instance of `Regex::MatchData` if it matched, `nil`
# otherwise.
#
# When matching a regular expression using `=~` (either `String#=~` or
# `Regex#=~`), the returned value will be the index of the first match in the
# string if the expression matched, `nil` otherwise.
#
# ```
# /stack/ =~ "haystack"  # => 3
# "haystack" =~ /stack/  # => 3
# $~                     # => #<Regex::MatchData "stack">
# /needle/ =~ "haystack" # => nil
# "haystack" =~ /needle/ # => nil
# $~                     # raises Exception
# ```
#
# When matching a regular expression using `#match` (either `String#match` or
# `Regex#match`), the returned value will be a `Regex::MatchData` if the expression
# matched, `nil` otherwise.
#
# ```
# /hay/.match("haystack")    # => #<Regex::MatchData "hay">
# "haystack".match(/hay/)    # => #<Regex::MatchData "hay">
# $~                         # => #<Regex::MatchData "hay">
# /needle/.match("haystack") # => nil
# "haystack".match(/needle/) # => nil
# $~                         # raises Exception
# ```
#
# [Regular expressions](https://en.wikipedia.org/wiki/Regular_expression)
# have their own language for describing strings.
#
# Many programming languages and tools implement their own regular expression
# language, but Crystal uses [PCRE](http://www.pcre.org/), a popular C library
# for providing regular expressions. Here give a brief summary of the most
# basic features of regular expressions - grouping, repetition, and
# alternation - but the feature set of PCRE extends far beyond these, and we
# don't attempt to describe it in full here. For more information, refer to
# the PCRE documentation, especially the
# [full pattern syntax](http://www.pcre.org/original/doc/html/pcrepattern.html)
# or
# [syntax quick reference](http://www.pcre.org/original/doc/html/pcresyntax.html).
#
# The regular expression language can be used to match much more than just the
# static substrings in the above examples. Certain characters, called
# [metacharacters](http://www.pcre.org/original/doc/html/pcrepattern.html#SEC4),
# are given special treatment in regular expressions, and can be used to
# describe more complex patterns. To match metacharacters literally in a
# regular expression, they must be escaped by being preceded with a backslash
# (`\`). `escape` will do this automatically for a given String.
#
# A group of characters (often called a capture group or
# [subpattern](http://www.pcre.org/original/doc/html/pcrepattern.html#SEC14))
# can be identified by enclosing it in parentheses (`()`). The contents of
# each capture group can be extracted on a successful match:
#
# ```
# /a(sd)f/.match("_asdf_")                     # => #<Regex::MatchData "asdf" 1:"sd">
# /a(sd)f/.match("_asdf_").try &.[1]           # => "sd"
# /a(?<grp>sd)f/.match("_asdf_")               # => #<Regex::MatchData "asdf" grp:"sd">
# /a(?<grp>sd)f/.match("_asdf_").try &.["grp"] # => "sd"
# ```
#
# Capture groups are indexed starting from 1. Methods that accept a capture
# group index will usually also accept 0 to refer to the full match. Capture
# groups can also be given names, using the `(?&lt;name&gt;...)` syntax, as in the
# previous example.
#
# Following a match, the special variables $N (e.g., $1, $2, $3, ...) can be used
# to access a capture group. Trying to access an invalid capture group will raise an
# exception. Note that it is possible to have a successful match with a nil capture:
#
# ```
# /(spice)(s)?/.match("spice") # => #<Regex::MatchData "spice" 1:"spice" 2:nil>
# $1                           # => "spice"
# $2                           # => raises Exception
# ```
#
# This can be mitigated by using the nilable version of the above: $N?,
# (e.g., $1? $2?, $3?, ...). Changing the above to use `$2?` instead of `$2`
# would return `nil`. `$2?.nil?` would return `true`.
#
# A character or group can be
# [repeated](http://www.pcre.org/original/doc/html/pcrepattern.html#SEC17)
# or made optional using an asterisk (`*` - zero or more), a plus sign
# (`+` - one or more), integer bounds in curly braces
# (`{n,m}`) (at least `n`, no more than `m`), or a question mark
# (`?`) (zero or one).
#
# ```
# /fo*/.match("_f_")         # => #<Regex::MatchData "f">
# /fo+/.match("_f_")         # => nil
# /fo*/.match("_foo_")       # => #<Regex::MatchData "foo">
# /fo{3,}/.match("_foo_")    # => nil
# /fo{1,3}/.match("_foo_")   # => #<Regex::MatchData "foo">
# /fo*/.match("_foo_")       # => #<Regex::MatchData "foo">
# /fo*/.match("_foooooooo_") # => #<Regex::MatchData "foooooooo">
# /fo{,3}/.match("_foooo_")  # => nil
# /f(op)*/.match("fopopo")   # => #<Regex::MatchData "fopop" 1: "op">
# /foo?bar/.match("foobar")  # => #<Regex::MatchData "foobar">
# /foo?bar/.match("fobar")   # => #<Regex::MatchData "fobar">
# ```
#
# Alternatives can be separated using a
# [vertical bar](http://www.pcre.org/original/doc/html/pcrepattern.html#SEC12)
# (`|`). Any single character can be represented by
# [dot](http://www.pcre.org/original/doc/html/pcrepattern.html#SEC7)
# (`.`). When matching only one character, specific
# alternatives can be expressed as a
# [character class](http://www.pcre.org/original/doc/html/pcrepattern.html#SEC9),
# enclosed in square brackets (`[]`):
#
# ```
# /foo|bar/.match("foo")     # => #<Regex::MatchData "foo">
# /foo|bar/.match("bar")     # => #<Regex::MatchData "bar">
# /_(x|y)_/.match("_x_")     # => #<Regex::MatchData "_x_" 1: "x">
# /_(x|y)_/.match("_y_")     # => #<Regex::MatchData "_y_" 1: "y">
# /_(x|y)_/.match("_(x|y)_") # => nil
# /_(x|y)_/.match("_(x|y)_") # => nil
# /_._/.match("_x_")         # => #<Regex::MatchData "_x_">
# /_[xyz]_/.match("_x_")     # => #<Regex::MatchData "_x_">
# /_[a-z]_/.match("_x_")     # => #<Regex::MatchData "_x_">
# /_[^a-z]_/.match("_x_")    # => nil
# /_[^a-wy-z]_/.match("_x_") # => #<Regex::MatchData "_x_">
# ```
#
# Regular expressions can be defined with these 3
# [optional flags](http://www.pcre.org/original/doc/html/pcreapi.html#SEC11):
#
# * `i`: ignore case (PCRE_CASELESS)
# * `m`: multiline (PCRE_MULTILINE and PCRE_DOTALL)
# * `x`: extended (PCRE_EXTENDED)
#
# ```
# /asdf/ =~ "ASDF"    # => nil
# /asdf/i =~ "ASDF"   # => 0
# /^z/i =~ "ASDF\nZ"  # => nil
# /^z/im =~ "ASDF\nZ" # => 5
# ```
#
# PCRE supports other encodings, but Crystal strings are UTF-8 only, so Crystal
# regular expressions are also UTF-8 only (by default).
#
# PCRE optionally permits named capture groups (named subpatterns) to not be
# unique. Crystal exposes the name table of a `Regex` as a
# `Hash` of `String` => `Int32`, and therefore requires named capture groups to have
# unique names within a single `Regex`.
class Regex
  @[Flags]
  enum Options
    IGNORE_CASE = 1
    # PCRE native `PCRE_MULTILINE` flag is `2`, and `PCRE_DOTALL` is `4`
    # - `PCRE_DOTALL` changes the "`.`" meaning
    # - `PCRE_MULTILINE` changes "`^`" and "`$`" meanings
    #
    # Crystal modifies this meaning to have essentially one unique "`m`"
    # flag that activates both behaviours, so here we do the same by
    # mapping `MULTILINE` to `PCRE_MULTILINE | PCRE_DOTALL`.
    MULTILINE = 6
    EXTENDED  = 8
    # :nodoc:
    ANCHORED = 16
    # :nodoc:
    UTF_8 = 0x00000800
    # :nodoc:
    NO_UTF8_CHECK = 0x00002000
    # :nodoc:
    DUPNAMES = 0x00080000
  end

  # Return a `Regex::Options` representing the optional flags applied to this `Regex`.
  #
  # ```
  # /ab+c/ix.options      # => Regex::Options::IGNORE_CASE | Regex::Options::EXTENDED
  # /ab+c/ix.options.to_s # => "IGNORE_CASE, EXTENDED"
  # ```
  getter options : Options

  # Return the original `String` representation of the `Regex` pattern.
  #
  # ```
  # /ab+c/x.source # => "ab+c"
  # ```
  getter source : String

  # Creates a new `Regex` out of the given source `String`.
  #
  # ```
  # Regex.new("^a-z+:\\s+\\w+")                   # => /^a-z+:\s+\w+/
  # Regex.new("cat", Regex::Options::IGNORE_CASE) # => /cat/i
  # options = Regex::Options::IGNORE_CASE | Regex::Options::EXTENDED
  # Regex.new("dog", options) # => /dog/ix
  # ```
  def initialize(source : String, @options : Options = Options::None)
    # PCRE's pattern must have their null characters escaped
    source = source.gsub('\u{0}', "\\0")
    @source = source

    @re = LibPCRE.compile(@source, (options | Options::UTF_8 | Options::NO_UTF8_CHECK | Options::DUPNAMES), out errptr, out erroffset, nil)
    raise ArgumentError.new("#{String.new(errptr)} at #{erroffset}") if @re.null?
    @extra = LibPCRE.study(@re, 0, out studyerrptr)
    raise ArgumentError.new("#{String.new(studyerrptr)}") if @extra.null? && studyerrptr
    LibPCRE.full_info(@re, nil, LibPCRE::INFO_CAPTURECOUNT, out @captures)
  end

  # Determines Regex's source validity. If it is, `nil` is returned.
  # If it's not, a `String` containing the error message is returned.
  #
  # ```
  # Regex.error?("(foo|bar)") # => nil
  # Regex.error?("(foo|bar")  # => "missing ) at 8"
  # ```
  def self.error?(source)
    re = LibPCRE.compile(source, (Options::UTF_8 | Options::NO_UTF8_CHECK | Options::DUPNAMES), out errptr, out erroffset, nil)
    if re
      nil
    else
      "#{String.new(errptr)} at #{erroffset}"
    end
  end

  # Returns a `String` constructed by escaping any metacharacters in *str*.
  #
  # ```
  # string = Regex.escape("\*?{}.") # => "\\*\\?\\{\\}\\."
  # /#{string}/                     # => /\*\?\{\}\./
  # ```
  def self.escape(str) : String
    String.build do |result|
      str.each_byte do |byte|
        case byte.unsafe_chr
        when ' ', '.', '\\', '+', '*', '?', '[',
             '^', ']', '$', '(', ')', '{', '}',
             '=', '!', '<', '>', '|', ':', '-'
          result << '\\'
          result.write_byte byte
        else
          result.write_byte byte
        end
      end
    end
  end

  # Union. Returns a `Regex` that matches any of *patterns*.
  #
  # All capture groups in the patterns after the first one will have their
  # indexes offset.
  #
  # ```
  # re = Regex.union([/skiing/i, "sledding"])
  # re.match("Skiing")   # => #<Regex::MatchData "Skiing">
  # re.match("sledding") # => #<Regex::MatchData "sledding">
  # re = Regex.union({/skiing/i, "sledding"})
  # re.match("Skiing")   # => #<Regex::MatchData "Skiing">
  # re.match("sledding") # => #<Regex::MatchData "sledding">
  # ```
  def self.union(patterns : Enumerable(Regex | String)) : self
    new patterns.map { |pattern| union_part pattern }.join("|")
  end

  # Union. Returns a `Regex` that matches any of *patterns*.
  #
  # All capture groups in the patterns after the first one will have their
  # indexes offset.
  #
  # ```
  # re = Regex.union(/skiing/i, "sledding")
  # re.match("Skiing")   # => #<Regex::MatchData "Skiing">
  # re.match("sledding") # => #<Regex::MatchData "sledding">
  # ```
  def self.union(*patterns : Regex | String) : self
    union patterns
  end

  private def self.union_part(pattern : Regex)
    pattern.to_s
  end

  private def self.union_part(pattern : String)
    escape pattern
  end

  # Union. Returns a `Regex` that matches either of the operands.
  #
  # All capture groups in the second operand will have their indexes
  # offset.
  #
  # ```
  # re = /skiing/i + /sledding/
  # re.match("Skiing")   # => #<Regex::MatchData "Skiing">
  # re.match("sledding") # => #<Regex::MatchData "sledding">
  # ```
  def +(other)
    Regex.union(self, other)
  end

  # Equality. Two regexes are equal if their sources and options are the same.
  #
  # ```
  # /abc/ == /abc/i  # => false
  # /abc/i == /ABC/i # => false
  # /abc/i == /abc/i # => true
  # ```
  def ==(other : Regex)
    source == other.source && options == other.options
  end

  # Case equality. This is equivalent to `#match` or `#=~` but only returns
  # `true` or `false`. Used in `case` expressions. The special variable
  # `$~` will contain a `Regex::MatchData` if there was a match, `nil` otherwise.
  #
  # ```
  # a = "HELLO"
  # b = case a
  #     when /^[a-z]*$/
  #       "Lower case"
  #     when /^[A-Z]*$/
  #       "Upper case"
  #     else
  #       "Mixed case"
  #     end
  # b # => "Upper case"
  # ```
  def ===(other : String)
    match = match(other)
    $~ = match
    !match.nil?
  end

  # Match. Matches a regular expression against *other* and returns
  # the starting position of the match if *other* is a matching `String`,
  # otherwise `nil`. `$~` will contain a `Regex::MatchData` if there was a match,
  # `nil` otherwise.
  #
  # ```
  # /at/ =~ "input data" # => 7
  # /ax/ =~ "input data" # => nil
  # ```
  def =~(other : String)
    match = self.match(other)
    $~ = match
    match.try &.begin(0)
  end

  # Match. When the argument is not a `String`, always returns `nil`.
  #
  # ```
  # /at/ =~ "input data" # => 7
  # /ax/ =~ "input data" # => nil
  # ```
  def =~(other)
    nil
  end

  # Convert to `String` in literal format. Returns the source as a `String` in
  # Regex literal format, delimited in forward slashes (`/`), with any
  # optional flags included.
  #
  # ```
  # /ab+c/ix.inspect # => "/ab+c/ix"
  # ```
  def inspect(io : IO)
    io << "/"
    append_source(io)
    io << "/"
    io << "i" if options.ignore_case?
    io << "m" if options.multiline?
    io << "x" if options.extended?
  end

  # Match at character index. Matches a regular expression against `String`
  # *str*. Starts at the character index given by *pos* if given, otherwise at
  # the start of *str*. Returns a `Regex::MatchData` if *str* matched, otherwise
  # `nil`. `$~` will contain the same value that was returned.
  #
  # ```
  # /(.)(.)(.)/.match("abc").try &.[2]   # => "b"
  # /(.)(.)/.match("abc", 1).try &.[2]   # => "c"
  # /(.)(.)/.match("クリスタル", 3).try &.[2] # => "ル"
  # ```
  def match(str, pos = 0, options = Regex::Options::None) : MatchData?
    if byte_index = str.char_index_to_byte_index(pos)
      match = match_at_byte_index(str, byte_index, options)
    else
      match = nil
    end

    $~ = match
  end

  # Match at byte index. Matches a regular expression against `String`
  # *str*. Starts at the byte index given by *pos* if given, otherwise at
  # the start of *str*. Returns a `Regex::MatchData` if *str* matched, otherwise
  # `nil`. `$~` will contain the same value that was returned.
  #
  # ```
  # /(.)(.)(.)/.match_at_byte_index("abc").try &.[2]   # => "b"
  # /(.)(.)/.match_at_byte_index("abc", 1).try &.[2]   # => "c"
  # /(.)(.)/.match_at_byte_index("クリスタル", 3).try &.[2] # => "ス"
  # ```
  def match_at_byte_index(str, byte_index = 0, options = Regex::Options::None) : MatchData?
    return ($~ = nil) if byte_index > str.bytesize

    ovector_size = (@captures + 1) * 3
    ovector = Pointer(Int32).malloc(ovector_size)
    ret = LibPCRE.exec(@re, @extra, str, str.bytesize, byte_index, (options | Options::NO_UTF8_CHECK), ovector, ovector_size)
    if ret > 0
      match = MatchData.new(self, @re, str, byte_index, ovector, @captures)
    else
      match = nil
    end

    $~ = match
  end

  # Returns a `Hash` where the values are the names of capture groups and the
  # keys are their indexes. Non-named capture groups will not have entries in
  # the `Hash`. Capture groups are indexed starting from `1`.
  #
  # ```
  # /(.)/.name_table                         # => {}
  # /(?<foo>.)/.name_table                   # => {1 => "foo"}
  # /(?<foo>.)(?<bar>.)/.name_table          # => {2 => "bar", 1 => "foo"}
  # /(.)(?<foo>.)(.)(?<bar>.)(.)/.name_table # => {4 => "bar", 2 => "foo"}
  # ```
  def name_table : Hash(UInt16, String)
    LibPCRE.full_info(@re, @extra, LibPCRE::INFO_NAMECOUNT, out name_count)
    LibPCRE.full_info(@re, @extra, LibPCRE::INFO_NAMEENTRYSIZE, out name_entry_size)
    table_pointer = Pointer(UInt8).null
    LibPCRE.full_info(@re, @extra, LibPCRE::INFO_NAMETABLE, pointerof(table_pointer).as(Pointer(Int32)))
    name_table = table_pointer.to_slice(name_entry_size*name_count)

    lookup = Hash(UInt16, String).new

    name_count.times do |i|
      capture_offset = i * name_entry_size
      capture_number = (name_table[capture_offset].to_u16 << 8) | name_table[capture_offset + 1].to_u16

      name_offset = capture_offset + 2
      name = String.new((name_table + name_offset).pointer(name_entry_size - 3))

      lookup[capture_number] = name
    end

    lookup
  end

  # Convert to `String` in subpattern format. Produces a `String` which can be
  # embedded in another `Regex` via interpolation, where it will be interpreted
  # as a non-capturing subexpression in another regular expression.
  #
  # ```
  # re = /A*/i                 # => /A*/i
  # re.to_s                    # => "(?i-msx:A*)"
  # "Crystal".match(/t#{re}l/) # => #<Regex::MatchData "tal">
  # re = /A*/                  # => "(?-imsx:A*)"
  # "Crystal".match(/t#{re}l/) # => nil
  # ```
  def to_s(io : IO)
    io << "(?"
    io << "i" if options.ignore_case?
    io << "ms" if options.multiline?
    io << "x" if options.extended?

    io << "-"
    io << "i" unless options.ignore_case?
    io << "ms" unless options.multiline?
    io << "x" unless options.extended?

    io << ":"
    append_source(io)
    io << ")"
  end

  private def append_source(io)
    source.each_char do |char|
      if char == '/'
        io << "\\/"
      else
        io << char
      end
    end
  end

  def dup
    self
  end

  def clone
    self
  end
end