ruby--ruby/doc/re.rb

module Doc
#  Regular expressions (<i>regexp</i>s) are patterns which describe the
#  contents of a string. They're used for testing whether a string contains a
#  given pattern, or extracting the portions that match. They are created
#  with the <tt>/</tt><i>pat</i><tt>/</tt> and
#  <tt>%r{</tt><i>pat</i><tt>}</tt> literals or the <tt>Regexp.new</tt>
#  constructor.
#
#  A regexp is usually delimited with forward slashes (<tt>/</tt>). For
#  example:
#
#      /hay/ =~ 'haystack'   #=> 0
#      /y/.match('haystack') #=> #<MatchData "y">
#
#  If a string contains the pattern it is said to <i>match</i>. A literal
#  string matches itself.
#
#      # 'haystack' does not contain the pattern 'needle', so doesn't match.
#      /needle/.match('haystack') #=> nil
#      # 'haystack' does contain the pattern 'hay', so it matches
#      /hay/.match('haystack')    #=> #<MatchData "hay">
#
#  Specifically, <tt>/st/</tt> requires that the string contains the letter
#  _s_ followed by the letter _t_, so it matches _haystack_, also.
#
#  == Metacharacters and Escapes
#
#  The following are <i>metacharacters</i> <tt>(</tt>, <tt>)</tt>,
#  <tt>[</tt>, <tt>]</tt>, <tt>{</tt>, <tt>}</tt>, <tt>.</tt>, <tt>?</tt>,
#  <tt>+</tt>, <tt>*</tt>. They have a specific meaning when appearing in a
#  pattern. To match them literally they must be backslash-escaped. To match
#  a backslash literally backslash-escape that: <tt>\\\\\\</tt>.
#
#      /1 \+ 2 = 3\?/.match('Does 1 + 2 = 3?') #=> #<MatchData "1 + 2 = 3?">
#
#  Patterns behave like double-quoted strings so can contain the same
#  backslash escapes.
#
#      /\s\u{6771 4eac 90fd}/.match("Go to 東京都")
#          #=> #<MatchData " 東京都">
#
#  Arbitrary Ruby expressions can be embedded into patterns with the
#  <tt>#{...}</tt> construct.
#
#      place = "東京都"
#      /#{place}/.match("Go to 東京都")
#          #=> #<MatchData "東京都">
#
#  == Character Classes
#
#  A <i>character class</i> is delimited with square brackets (<tt>[</tt>,
#  <tt>]</tt>) and lists characters that may appear at that point in the
#  match. <tt>/[ab]/</tt> means _a_ or _b_, as opposed to <tt>/ab/</tt> which
#  means _a_ followed by _b_.
#
#      /W[aeiou]rd/.match("Word") #=> #<MatchData "Word">
#
#  Within a character class the hyphen (<tt>-</tt>) is a metacharacter
#  denoting an inclusive range of characters. <tt>[abcd]</tt> is equivalent
#  to <tt>[a-d]</tt>. A range can be followed by another range, so
#  <tt>[abcdwxyz]</tt> is equivalent to <tt>[a-dw-z]</tt>. The order in which
#  ranges or individual characters appear inside a character class is
#  irrelevant.
#
#      /[0-9a-f]/.match('9f') #=> #<MatchData "9">
#      /[9f]/.match('9f')     #=> #<MatchData "9">
#
#  If the first character of a character class is a caret (<tt>^</tt>) the
#  class is inverted: it matches any character _except_ those named.
#
#      /[^a-eg-z]/.match('f') #=> #<MatchData "f">
#
#  A character class may contain another character class. By itself this
#  isn't useful because <tt>[a-z[0-9]]</tt> describes the same set as
#  <tt>[a-z0-9]</tt>. However, character classes also support the <tt>&&</tt>
#  operator which performs set intersection on its arguments. The two can be
#  combined as follows:
#
#      /[a-w&&[^c-g]z]/ # ([a-w] AND ([^c-g] OR z))
#      # This is equivalent to:
#      /[abh-w]/
#
#  The following metacharacters also behave like character classes:
#
#  * <tt>/./</tt> - Any character except a newline.
#  * <tt>/./m</tt> - Any character (the +m+ modifier enables multiline mode)
#  * <tt>/\w/</tt> - A word character (<tt>[a-zA-Z0-9_]</tt>)
#  * <tt>/\W/</tt> - A non-word character (<tt>[^a-zA-Z0-9_]</tt>)
#  * <tt>/\d/</tt> - A digit character (<tt>[0-9]</tt>)
#  * <tt>/\D/</tt> - A non-digit character (<tt>[^0-9]</tt>)
#  * <tt>/\h/</tt> - A hexdigit character (<tt>[0-9a-fA-F]</tt>)
#  * <tt>/\H/</tt> - A non-hexdigit character (<tt>[^0-9a-fA-F]</tt>)
#  * <tt>/\s/</tt> - A whitespace character: <tt>/[ \t\r\n\f]/</tt>
#  * <tt>/\S/</tt> - A non-whitespace character: <tt>/[^ \t\r\n\f]/</tt>
#
#  POSIX <i>bracket expressions</i> are also similar to character classes.
#  They provide a portable alternative to the above, with the added benefit
#  that they encompass non-ASCII characters. For instance, <tt>/\d/</tt>
#  matches only the ASCII decimal digits (0-9); whereas <tt>/[[:digit:]]/</tt>
#  matches any character in the Unicode _Nd_ category.
#
#  * <tt>/[[:alnum:]]/</tt> - Alphabetic and numeric character
#  * <tt>/[[:alpha:]]/</tt> - Alphabetic character
#  * <tt>/[[:blank:]]/</tt> - Space or tab
#  * <tt>/[[:cntrl:]]/</tt> - Control character
#  * <tt>/[[:digit:]]/</tt> - Digit
#  * <tt>/[[:graph:]]/</tt> - Non-blank character (excludes spaces, control
#    characters, and similar)
#  * <tt>/[[:lower:]]/</tt> - Lowercase alphabetical character
#  * <tt>/[[:print:]]/</tt> - Like [:graph:], but includes the space character
#  * <tt>/[[:punct:]]/</tt> - Punctuation character
#  * <tt>/[[:space:]]/</tt> - Whitespace character (<tt>[:blank:]</tt>, newline,
#     carriage return, etc.)
#  * <tt>/[[:upper:]]/</tt> - Uppercase alphabetical
#  * <tt>/[[:xdigit:]]/</tt> - Digit allowed in a hexadecimal number (i.e.,
#    0-9a-fA-F)
#
#  Ruby also supports the following non-POSIX character classes:
#
#  * <tt>/[[:word:]]/</tt> - A character in one of the following Unicode
#    general categories _Letter_, _Mark_, _Number_,
#    <i>Connector_Punctuation<i/i>
#  * <tt>/[[:ascii:]]/</tt> - A character in the ASCII character set
#
#      # U+06F2 is "EXTENDED ARABIC-INDIC DIGIT TWO"
#      /[[:digit:]]/.match("\u06F2")    #=> #<MatchData "\u{06F2}">
#      /[[:upper:]][[:lower:]]/.match("Hello") #=> #<MatchData "He">
#      /[[:xdigit:]][[:xdigit:]]/.match("A6")  #=> #<MatchData "A6">
#
#  == Repetition
#
#  The constructs described so far match a single character. They can be
#  followed by a repetition metacharacter to specify how many times they need
#  to occur. Such metacharacters are called <i>quantifiers</i>.
#
#  * <tt>*</tt> - Zero or more times
#  * <tt>+</tt> - One or more times
#  * <tt>?</tt> - Zero or one times (optional)
#  * <tt>{</tt><i>n</i><tt>}</tt> - Exactly <i>n</i> times
#  * <tt>{</tt><i>n</i><tt>,}</tt> - <i>n</i> or more times
#  * <tt>{,</tt><i>m</i><tt>}</tt> - <i>m</i> or less times
#  * <tt>{</tt><i>n</i><tt>,</tt><i>m</i><tt>}</tt> - At least <i>n</i> and
#    at most <i>m</i> times
#
#      # At least one uppercase character ('H'), at least one lowercase
#      # character ('e'), two 'l' characters, then one 'o'
#      "Hello".match(/[[:upper:]]+[[:lower:]]+l{2}o/) #=> #<MatchData "Hello">
#
#  Repetition is <i>greedy</i> by default: as many occurrences as possible
#  are matched while still allowing the overall match to succeed. By
#  contrast, <i>lazy</i> matching makes the minimal amount of matches
#  necessary for overall success. A greedy metacharacter can be made lazy by
#  following it with <tt>?</tt>.
#
#      # Both patterns below match the string. The fist uses a greedy
#      # quantifier so '.+' matches '<a><b>'; the second uses a lazy
#      # quantifier so '.+?' matches '<a>'.
#      /<.+>/.match("<a><b>")  #=> #<MatchData "<a><b>">
#      /<.+?>/.match("<a><b>") #=> #<MatchData "<a>">
#
#  A quantifier followed by <tt>+</tt> matches <i>possessively</i>: once it
#  has matched it does not backtrack. They behave like greedy quantifiers,
#  but having matched they refuse to "give up" their match even if this
#  jeopardises the overall match.
#
#  == Capturing
#
#  Parentheses can be used for <i>capturing</i>. The text enclosed by the
#  <i>n</i><sup>th</sup> group of parentheses can be subsequently referred to
#  with <i>n</i>. Within a pattern use the <i>backreference</i>
#  <tt>\</tt><i>n</i>; outside of the pattern use
#  <tt>MatchData[</tt><i>n</i><tt>]</tt>.
#
#      # 'at' is captured by the first group of parentheses, then referred to
#      # later with \1
#      /[csh](..) [csh]\1 in/.match("The cat sat in the hat")
#          #=> #<MatchData "cat sat in" 1:"at">
#      # Regexp#match returns a MatchData object which makes the captured
#      # text available with its #[] method.
#      /[csh](..) [csh]\1 in/.match("The cat sat in the hat")[1] #=> 'at'
#
#  Capture groups can be referred to by name when defined with the
#  <tt>(?<</tt><i>name</i><tt>>)</tt> or <tt>(?'</tt><i>name</i><tt>')</tt>
#  constructs.
#
#      /\$(?<dollars>\d+)\.(?<cents>\d+)/.match("$3.67")
#          => #<MatchData "$3.67" dollars:"3" cents:"67">
#      /\$(?<dollars>\d+)\.(?<cents>\d+)/.match("$3.67")[:dollars] #=> "3"
#
#  Named groups can be backreferenced with <tt>\k<</tt><i>name</i><tt>></tt>,
#  where _name_ is the group name.
#
#      /(?<vowel>[aeiou]).\k<vowel>.\k<vowel>/.match('ototomy')
#          #=> #<MatchData "ototo" vowel:"o">
#
#  *Note*: A regexp can't use named backreferences and numbered
#  backreferences simultaneously.
#
#  When named capture groups are used with a literal regexp on the left-hand
#  side of an expression and the <tt>=~</tt> operator, the captured text is
#  also assigned to local variables with corresponding names.
#
#      /\$(?<dollars>\d+)\.(?<cents>\d+)/ =~ "$3.67" #=> 0
#      dollars #=> "3"
#
#  == Grouping
#
#  Parentheses also <i>group</i> the terms they enclose, allowing them to be
#  quantified as one <i>atomic</i> whole.
#
#      # The pattern below matches a vowel followed by 2 word characters:
#      # 'aen'
#      /[aeiou]\w{2}/.match("Caenorhabditis elegans") #=> #<MatchData "aen">
#      # Whereas the following pattern matches a vowel followed by a word
#      # character, twice, i.e. <tt>[aeiou]\w[aeiou]\w</tt>: 'enor'.
#      /([aeiou]\w){2}/.match("Caenorhabditis elegans")
#          #=> #<MatchData "enor" 1:"or">
#
#  The <tt>(?:</tt>...<tt>)</tt> construct provides grouping without
#  capturing. That is, it combines the terms it contains into an atomic whole
#  without creating a backreference. This benefits performance at the slight
#  expense of readabilty.
#
#      # The group of parentheses captures 'n' and the second 'ti'. The
#      # second group is referred to later with the backreference \2
#      /I(n)ves(ti)ga\2ons/.match("Investigations")
#          #=> #<MatchData "Investigations" 1:"n" 2:"ti">
#      # The first group of parentheses is now made non-capturing with '?:',
#      # so it still matches 'n', but doesn't create the backreference. Thus,
#      # the backreference \1 now refers to 'ti'.
#      /I(?:n)ves(ti)ga\1ons/.match("Investigations")
#          #=> #<MatchData "Investigations" 1:"ti">
#
#  === Atomic Grouping
#
#  Grouping can be made <i>atomic</i> with
#  <tt>(?></tt><i>pat</i><tt>)</tt>. This causes the subexpression <i>pat</i>
#  to be matched independently of the rest of the expression such that what
#  it matches becomes fixed for the remainder of the match, unless the entire
#  subexpression must be abandoned and subsequently revisited. In this
#  way <i>pat</i> is treated as a non-divisible whole. Atomic grouping is
#  typically used to optimise patterns so as to prevent the regular
#  expression engine from backtracking needlesly.
#
#      # The <tt>"</tt> in the pattern below matches the first character of
#      # the string, then <tt>.*</tt> matches <i>Quote"</i>. This causes the
#      # overall match to fail, so the text matched by <tt>.*</tt> is
#      # backtracked by one position, which leaves the final character of the
#      # string available to match <tt>"</tt>
#            /".*"/.match('"Quote"')     #=> #<MatchData "\"Quote\"">
#      # If <tt>.*</tt> is grouped atomically, it refuses to backtrack
#      # <i>Quote"</i>, even though this means that the overall match fails
#      /"(?>.*)"/.match('"Quote"') #=> nil
#
#  == Subexpression Calls
#
#  The <tt>\g<</tt><i>name</i><tt>></tt> syntax matches the previous
#  subexpression named _name_, which can be a group name or number, again.
#  This differs from backreferences in that it re-executes the group rather
#  than simply trying to re-match the same text.
#
#      # Matches a <i>(</i> character and assigns it to the <tt>paren</tt>
#      # group, tries to call that the <tt>paren</tt> sub-expression again
#      # but fails, then matches a literal <i>)</i>.
#      /\A(?<paren>\(\g<paren>*\))*\z/ =~ '()'
#
#
#      /\A(?<paren>\(\g<paren>*\))*\z/ =~ '(())' #=> 0
#      # ^1
#      #      ^2
#      #           ^3
#      #                 ^4
#      #      ^5
#      #           ^6
#      #                      ^7
#      #                       ^8
#      #                       ^9
#      #                           ^10
#
#  1.  Matches at the beginning of the string, i.e. before the first
#      character.
#  2.  Enters a named capture group called <tt>paren</tt>
#  3.  Matches a literal <i>(</i>, the first character in the string
#  4.  Calls the <tt>paren</tt> group again, i.e. recurses back to the
#      second step
#  5.  Re-enters the <tt>paren</tt> group
#  6.  Matches a literal <i>(</i>, the second character in the
#      string
#  7.  Try to call <tt>paren</tt> a third time, but fail because
#      doing so would prevent an overall successful match
#  8.  Match a literal <i>)</i>, the third character in the string.
#      Marks the end of the second recursive call
#  9.  Match a literal <i>)</i>, the fourth character in the string
#  10. Match the end of the string
#
#  == Alternation
#
#  The vertical bar metacharacter (<tt>|</tt>) combines two expressions into
#  a single one that matches either of the expressions. Each expression is an
#  <i>alternative</i>.
#
#      /\w(and|or)\w/.match("Feliformia") #=> #<MatchData "form" 1:"or">
#      /\w(and|or)\w/.match("furandi")    #=> #<MatchData "randi" 1:"and">
#      /\w(and|or)\w/.match("dissemblance") #=> nil
#
#  == Character Properties
#
#  The <tt>\p{}</tt> construct matches characters with the named property,
#  much like POSIX bracket classes.
#
#  * <tt>/\p{Alnum}/</tt> - Alphabetic and numeric character
#  * <tt>/\p{Alpha}/</tt> - Alphabetic character
#  * <tt>/\p{Blank}/</tt> - Space or tab
#  * <tt>/\p{Cntrl}/</tt> - Control character
#  * <tt>/\p{Digit}/</tt> - Digit
#  * <tt>/\p{Graph}/</tt> - Non-blank character (excludes spaces, control
#    characters, and similar)
#  * <tt>/\p{Lower}/</tt> - Lowercase alphabetical character
#  * <tt>/\p{Print}/</tt> - Like <tt>\p{Graph}</tt>, but includes the space character
#  * <tt>/\p{Punct}/</tt> - Punctuation character
#  * <tt>/\p{Space}/</tt> - Whitespace character (<tt>[:blank:]</tt>, newline,
#    carriage return, etc.)
#  * <tt>/\p{Upper}/</tt> - Uppercase alphabetical
#  * <tt>/\p{XDigit}/</tt> - Digit allowed in a hexadecimal number (i.e., 0-9a-fA-F)
#  * <tt>/\p{Word}/</tt> - A member of one of the following Unicode general
#    category <i>Letter</i>, <i>Mark</i>, <i>Number</i>,
#    <i>Connector\_Punctuation</i>
#  * <tt>/\p{ASCII}/</tt> - A character in the ASCII character set
#  * <tt>/\p{Any}/</tt> - Any Unicode character (including unassigned
#    characters)
#  * <tt>/\p{Assigned}/</tt> - An assigned character
#
#  A Unicode character's <i>General Category</i> value can also be matched
#  with <tt>\p{</tt><i>Ab</i><tt>}</tt> where <i>Ab</i> is the category's
#  abbreviation as described below:
#
#  * <tt>/\p{L}/</tt> - 'Letter'
#  * <tt>/\p{Ll}/</tt> - 'Letter: Lowercase'
#  * <tt>/\p{Lm}/</tt> - 'Letter: Mark'
#  * <tt>/\p{Lo}/</tt> - 'Letter: Other'
#  * <tt>/\p{Lt}/</tt> - 'Letter: Titlecase'
#  * <tt>/\p{Lu}/</tt> - 'Letter: Uppercase
#  * <tt>/\p{Lo}/</tt> - 'Letter: Other'
#  * <tt>/\p{M}/</tt> - 'Mark'
#  * <tt>/\p{Mn}/</tt> - 'Mark: Nonspacing'
#  * <tt>/\p{Mc}/</tt> - 'Mark: Spacing Combining'
#  * <tt>/\p{Me}/</tt> - 'Mark: Enclosing'
#  * <tt>/\p{N}/</tt> - 'Number'
#  * <tt>/\p{Nd}/</tt> - 'Number: Decimal Digit'
#  * <tt>/\p{Nl}/</tt> - 'Number: Letter'
#  * <tt>/\p{No}/</tt> - 'Number: Other'
#  * <tt>/\p{P}/</tt> - 'Punctuation'
#  * <tt>/\p{Pc}/</tt> - 'Punctuation: Connector'
#  * <tt>/\p{Pd}/</tt> - 'Punctuation: Dash'
#  * <tt>/\p{Ps}/</tt> - 'Punctuation: Open'
#  * <tt>/\p{Pe}/</tt> - 'Punctuation: Close'
#  * <tt>/\p{Pi}/</tt> - 'Punctuation: Initial Quote'
#  * <tt>/\p{Pf}/</tt> - 'Punctuation: Final Quote'
#  * <tt>/\p{Po}/</tt> - 'Punctuation: Other'
#  * <tt>/\p{S}/</tt> - 'Symbol'
#  * <tt>/\p{Sm}/</tt> - 'Symbol: Math'
#  * <tt>/\p{Sc}/</tt> - 'Symbol: Currency'
#  * <tt>/\p{Sc}/</tt> - 'Symbol: Currency'
#  * <tt>/\p{Sk}/</tt> - 'Symbol: Modifier'
#  * <tt>/\p{So}/</tt> - 'Symbol: Other'
#  * <tt>/\p{Z}/</tt> - 'Separator'
#  * <tt>/\p{Zs}/</tt> - 'Separator: Space'
#  * <tt>/\p{Zl}/</tt> - 'Separator: Line'
#  * <tt>/\p{Zp}/</tt> - 'Separator: Paragraph'
#  * <tt>/\p{C}/</tt> - 'Other'
#  * <tt>/\p{Cc}/</tt> - 'Other: Control'
#  * <tt>/\p{Cf}/</tt> - 'Other: Format'
#  * <tt>/\p{Cn}/</tt> - 'Other: Not Assigned'
#  * <tt>/\p{Co}/</tt> - 'Other: Private Use'
#  * <tt>/\p{Cs}/</tt> - 'Other: Surrogate'
#
#  Lastly, <tt>\p{}</tt> matches a character's Unicode <i>script</i>. The
#  following scripts are supported: <i>Arabic</i>, <i>Armenian</i>,
#  <i>Balinese</i>, <i>Bengali</i>, <i>Bopomofo</i>, <i>Braille</i>,
#  <i>Buginese</i>, <i>Buhid</i>, <i>Canadian_Aboriginal</i>, <i>Carian</i>,
#  <i>Cham</i>, <i>Cherokee</i>, <i>Common</i>, <i>Coptic</i>,
#  <i>Cuneiform</i>, <i>Cypriot</i>, <i>Cyrillic</i>, <i>Deseret</i>,
#  <i>Devanagari</i>, <i>Ethiopic</i>, <i>Georgian</i>, <i>Glagolitic</i>,
#  <i>Gothic</i>, <i>Greek</i>, <i>Gujarati</i>, <i>Gurmukhi</i>, <i>Han</i>,
#  <i>Hangul</i>, <i>Hanunoo</i>, <i>Hebrew</i>, <i>Hiragana</i>,
#  <i>Inherited</i>, <i>Kannada</i>, <i>Katakana</i>, <i>Kayah_Li</i>,
#  <i>Kharoshthi</i>, <i>Khmer</i>, <i>Lao</i>, <i>Latin</i>, <i>Lepcha</i>,
#  <i>Limbu</i>, <i>Linear_B</i>, <i>Lycian</i>, <i>Lydian</i>,
#  <i>Malayalam</i>, <i>Mongolian</i>, <i>Myanmar</i>, <i>New_Tai_Lue</i>,
#  <i>Nko</i>, <i>Ogham</i>, <i>Ol_Chiki</i>, <i>Old_Italic</i>,
#  <i>Old_Persian</i>, <i>Oriya</i>, <i>Osmanya</i>, <i>Phags_Pa</i>,
#  <i>Phoenician</i>, <i>Rejang</i>, <i>Runic</i>, <i>Saurashtra</i>,
#  <i>Shavian</i>, <i>Sinhala</i>, <i>Sundanese</i>, <i>Syloti_Nagri</i>,
#  <i>Syriac</i>, <i>Tagalog</i>, <i>Tagbanwa</i>, <i>Tai_Le</i>,
#  <i>Tamil</i>, <i>Telugu</i>, <i>Thaana</i>, <i>Thai</i>, <i>Tibetan</i>,
#  <i>Tifinagh</i>, <i>Ugaritic</i>, <i>Vai</i>, and <i>Yi</i>.
#
#      # Unicode codepoint U+06E9 is named "ARABIC PLACE OF SAJDAH" and
#      # belongs to the Arabic script.
#      /\p{Arabic}/.match("\u06E9") #=> #<MatchData "\u06E9">
#
#  All character properties can be inverted by prefixing their name with a
#  caret (<tt>^</tt>).
#
#      # Letter 'A' is not in the Unicode Ll (Letter; Lowercase) category, so
#      # this match succeeds
#      /\p{^Ll}/.match("A") #=> #<MatchData "A">
#
#  == Anchors
#
#  Anchors are metacharacter that match the zero-width positions between
#  characters, <i>anchoring</i> the match to a specific position.
#
#  * <tt>^</tt> - Matches beginning of line
#  * <tt>$</tt> - Matches end of line
#  * <tt>\A</tt> - Matches beginning of string.
#  * <tt>\Z</tt> - Matches end of string. If string ends with a newline,
#    it matches just before newline
#  * <tt>\z</tt> - Matches end of string
#  * <tt>\G</tt> - Matches point where last match finished
#  * <tt>\b</tt> - Matches word boundaries when outside brackets; backspace
#     (0x08) inside brackets
#  * <tt>\B</tt> - Matches non-word boundaries
#  * <tt>(?=</tt><i>pat</i><tt>)</tt> - <i>Positive lookahead</i> assertion:
#    ensures that the following characters match <i>pat</i>, but doesn't
#    include those characters in the matched text
#  * <tt>(?!</tt><i>pat</i><tt>)</tt> - <i>Negative lookahead</i> assertion:
#    ensures that the following characters do not match <i>pat</i>, but
#    doesn't include those characters in the matched text
#  * <tt>(?<=</tt><i>pat</i><tt>)</tt> - <i>Positive lookbehind</i>
#    assertion: ensures that the preceding characters match <i>pat</i>, but
#    doesn't include those characters in the matched text
#  * <tt>(?<!</tt><i>pat</i><tt>)</tt> - <i>Negative lookbehind</i>
#    assertion: ensures that the preceding characters do not match
#    <i>pat</i>, but doesn't include those characters in the matched text
#
#      # If a pattern isn't anchored it can begin at any point in the string
#      /real/.match("surrealist") #=> #<MatchData "real">
#      # Anchoring the pattern to the beginning of the string forces the
#      # match to start there. 'real' doesn't occur at the beginning of the
#      # string, so now the match fails
#      /\Areal/.match("surrealist") #=> nil
#      # The match below fails because although 'Demand' contains 'and', the
#      pattern does not occur at a word boundary.
#      /\band/.match("Demand")
#      # Whereas in the following example 'and' has been anchored to a
#      # non-word boundary so instead of matching the first 'and' it matches
#      # from the fourth letter of 'demand' instead
#      /\Band.+/.match("Supply and demand curve") #=> #<MatchData "and curve">
#      # The pattern below uses positive lookahead and positive lookbehind to
#      # match text appearing in <b></b> tags without including the tags in the
#      # match
#      /(?<=<b>)\w+(?=<\/b>)/.match("Fortune favours the <b>bold</b>")
#          #=> #<MatchData "bold">
#
#  == Options
#
#  The end delimiter for a regexp can be followed by one or more single-letter
#  options which control how the pattern can match.
#
#  * <tt>/pat/i</tt> - Ignore case
#  * <tt>/pat/m</tt> - Treat a newline as a character matched by <tt>.</tt>
#  * <tt>/pat/x</tt> - Ignore whitespace and comments in the pattern
#  * <tt>/pat/o</tt> - Perform <tt>#{}</tt> interpolation only once
#
#  <tt>i</tt>, <tt>m</tt>, and <tt>x</tt> can also be applied on the
#  subexpression level with the
#  <tt>(?</tt><i>on</i><tt>-</tt><i>off</i><tt>)</tt> construct, which
#  enables options <i>on</i>, and disables options <i>off</i> for the
#  expression enclosed by the parentheses.
#
#      /a(?i:b)c/.match('aBc') #=> #<MatchData "aBc">
#      /a(?i:b)c/.match('abc') #=> #<MatchData "abc">
#
#  == Free-Spacing Mode and Comments
#
#  As mentioned above, the <tt>x</tt> option enables <i>free-spacing</i>
#  mode. Literal white space inside the pattern is ignored, and the
#  octothorpe (<tt>#</tt>) character introduces a comment until the end of
#  the line. This allows the components of the pattern to be organised in a
#  potentially more readable fashion.
#
#      # A contrived pattern to match a number with optional decimal places
#      float_pat = /\A
#          [[:digit:]]+ # 1 or more digits before the decimal point
#          (\.          # Decimal point
#              [[:digit:]]+ # 1 or more digits after the decimal point
#          )? # The decimal point and following digits are optional
#      \Z/x
#      float_pat.match('3.14') #=> #<MatchData "3.14" 1:".14">
#
#  *Note*: To match whitespace in an <tt>x</tt> pattern use an escape such as
#  <tt>\s</tt> or <tt>\p{Space}</tt>.
#
#  Comments can be included in a non-<tt>x</tt> pattern with the
#  <tt>(?#</tt><i>comment</i><tt>)</tt> construct, where <i>comment</i> is
#  arbitrary text ignored by the regexp engine.
#
#  == Encoding
#
#  Regular expressions are assumed to use the source encoding. This can be
#  overridden with one of the following modifiers.
#
#  * <tt>/</tt><i>pat</i><tt>/u</tt> - UTF-8
#  * <tt>/</tt><i>pat</i><tt>/e</tt> - EUC-JP
#  * <tt>/</tt><i>pat</i><tt>/s</tt> - Windows-31J
#  * <tt>/</tt><i>pat</i><tt>/n</tt> - ASCII-8BIT
#
#  A regexp can be matched against a string when they either share an
#  encoding, or the regexp's encoding is _US-ASCII_ and the string's encoding
#  is ASCII-compatible.
#
#  If a match between incompatible encodings is attempted an
#  <tt>Encoding::CompatibilityError</tt> exception is raised.
#
#  The <tt>Regexp#fixed_encoding?</tt> predicate indicates whether the regexp
#  has a <i>fixed</i> encoding, that is one incompatible with ASCII. A
#  regexp's encoding can be explicitly fixed by supplying
#  <tt>Regexp::FIXEDENCODING</tt> as the second argument of
#  <tt>Regexp.new</tt>:
#
#      r = Regexp.new("a".force_encoding("iso-8859-1"),Regexp::FIXEDENCODING)
#      r =~"a\u3042"
#         #=> Encoding::CompatibilityError: incompatible encoding regexp match
#              (ISO-8859-1 regexp with UTF-8 string)
#
#  == Performance
#
#  Certain pathological combinations of constructs can lead to abysmally bad
#  performance.
#
#  Consider a string of 25 <i>a</i>s, a <i>d</i>, 4 <i>a</i>s, and a
#  <i>c</i>.
#
#      s = 'a' * 25 + 'd' 'a' * 4 + 'c'
#          #=> "aaaaaaaaaaaaaaaaaaaaaaaaadadadadac"
#
#  The following patterns match instantly as you would expect:
#
#      /(b|a)/ =~ s #=> 0
#      /(b|a+)/ =~ s #=> 0
#      /(b|a+)*\/ =~ s #=> 0
#
#  However, the following pattern takes appreciably longer:
#
#      /(b|a+)*c/ =~ s #=> 32
#
#  This happens because an atom in the regexp is quantified by both an
#  immediate <tt>+</tt> and an enclosing <tt>*</tt> with nothing to
#  differentiate which is in control of any particular character. The
#  nondeterminism that results produces super-linear performance. (Consult
#  <i>Mastering Regular Expressions</i> (3rd ed.), pp 222, by
#  <i>Jeffery Friedl</i>, for an in-depth analysis). This particular case
#  can be fixed by use of atomic grouping, which prevents the unnecessary
#  backtracking:
#
#      (start = Time.now) && /(b|a+)*c/ =~ s && (Time.now - start)
#         #=> 24.702736882
#      (start = Time.now) && /(?>b|a+)*c/ =~ s && (Time.now - start)
#         #=> 0.000166571
#
#  A similar case is typified by the following example, which takes
#  approximately 60 seconds to execute for me:
#
#      # Match a string of 29 <i>a</i>s against a pattern of 29 optional
#      # <i>a</i>s followed by 29 mandatory <i>a</i>s.
#      Regexp.new('a?' * 29 + 'a' * 29) =~ 'a' * 29
#
#  The 29 optional <i>a</i>s match the string, but this prevents the 29
#  mandatory <i>a</i>s that follow from matching. Ruby must then backtrack
#  repeatedly so as to satisfy as many of the optional matches as it can
#  while still matching the mandatory 29. It is plain to us that none of the
#  optional matches can succeed, but this fact unfortunately eludes Ruby.
#
#  One approach for improving performance is to anchor the match to the
#  beginning of the string, thus significantly reducing the amount of
#  backtracking needed.
#
#      Regexp.new('\A' 'a?' * 29 + 'a' * 29).match('a' * 29)
#          #=> #<MatchData "aaaaaaaaaaaaaaaaaaaaaaaaaaaaa">
#
#
  class Regexp; end
end