From d7f76f84d923ac8c7ef585e1f43fc0d691625377 Mon Sep 17 00:00:00 2001 From: nobu Date: Fri, 18 Sep 2009 01:11:55 +0000 Subject: [PATCH] * doc/re.rdoc: use rdoc mode. git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@24992 b2dd03c8-39d4-4d8f-98ff-823fe69b080e --- ChangeLog | 4 +- doc/re.rdoc | 1165 ++++++++++++++++++++++++++------------------------- 2 files changed, 586 insertions(+), 583 deletions(-) diff --git a/ChangeLog b/ChangeLog index 1ed763cc9f..65dafbfb64 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,4 +1,6 @@ -Fri Sep 18 10:11:27 2009 Nobuyoshi Nakada +Fri Sep 18 10:11:53 2009 Nobuyoshi Nakada + + * doc/re.rdoc: use rdoc mode. * misc/rdoc-mode.el: added. diff --git a/doc/re.rdoc b/doc/re.rdoc index 9671a7bd0b..6ca3325682 100644 --- a/doc/re.rdoc +++ b/doc/re.rdoc @@ -1,583 +1,584 @@ -# -*- coding: utf-8 -*- -# Regular expressions (regexps) 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 /pat/ and -# %r{pat} literals or the Regexp.new -# constructor. -# -# A regexp is usually delimited with forward slashes (/). For -# example: -# -# /hay/ =~ 'haystack' #=> 0 -# /y/.match('haystack') #=> # -# -# If a string contains the pattern it is said to match. 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') #=> # -# -# Specifically, /st/ requires that the string contains the letter -# _s_ followed by the letter _t_, so it matches _haystack_, also. -# -# == Metacharacters and Escapes -# -# The following are metacharacters (, ), -# [, ], {, }, ., ?, -# +, *. 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: \\\\\\. -# -# /1 \+ 2 = 3\?/.match('Does 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") -# #=> # -# -# Arbitrary Ruby expressions can be embedded into patterns with the -# #{...} construct. -# -# place = "東京都" -# /#{place}/.match("Go to 東京都") -# #=> # -# -# == Character Classes -# -# A character class is delimited with square brackets ([, -# ]) and lists characters that may appear at that point in the -# match. /[ab]/ means _a_ or _b_, as opposed to /ab/ which -# means _a_ followed by _b_. -# -# /W[aeiou]rd/.match("Word") #=> # -# -# Within a character class the hyphen (-) is a metacharacter -# denoting an inclusive range of characters. [abcd] is equivalent -# to [a-d]. A range can be followed by another range, so -# [abcdwxyz] is equivalent to [a-dw-z]. The order in which -# ranges or individual characters appear inside a character class is -# irrelevant. -# -# /[0-9a-f]/.match('9f') #=> # -# /[9f]/.match('9f') #=> # -# -# If the first character of a character class is a caret (^) the -# class is inverted: it matches any character _except_ those named. -# -# /[^a-eg-z]/.match('f') #=> # -# -# A character class may contain another character class. By itself this -# isn't useful because [a-z[0-9]] describes the same set as -# [a-z0-9]. However, character classes also support the && -# 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: -# -# * /./ - Any character except a newline. -# * /./m - Any character (the +m+ modifier enables multiline mode) -# * /\w/ - A word character ([a-zA-Z0-9_]) -# * /\W/ - A non-word character ([^a-zA-Z0-9_]) -# * /\d/ - A digit character ([0-9]) -# * /\D/ - A non-digit character ([^0-9]) -# * /\h/ - A hexdigit character ([0-9a-fA-F]) -# * /\H/ - A non-hexdigit character ([^0-9a-fA-F]) -# * /\s/ - A whitespace character: /[ \t\r\n\f]/ -# * /\S/ - A non-whitespace character: /[^ \t\r\n\f]/ -# -# POSIX bracket expressions 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, /\d/ -# matches only the ASCII decimal digits (0-9); whereas /[[:digit:]]/ -# matches any character in the Unicode _Nd_ category. -# -# * /[[:alnum:]]/ - Alphabetic and numeric character -# * /[[:alpha:]]/ - Alphabetic character -# * /[[:blank:]]/ - Space or tab -# * /[[:cntrl:]]/ - Control character -# * /[[:digit:]]/ - Digit -# * /[[:graph:]]/ - Non-blank character (excludes spaces, control -# characters, and similar) -# * /[[:lower:]]/ - Lowercase alphabetical character -# * /[[:print:]]/ - Like [:graph:], but includes the space character -# * /[[:punct:]]/ - Punctuation character -# * /[[:space:]]/ - Whitespace character ([:blank:], newline, -# carriage return, etc.) -# * /[[:upper:]]/ - Uppercase alphabetical -# * /[[:xdigit:]]/ - Digit allowed in a hexadecimal number (i.e., -# 0-9a-fA-F) -# -# Ruby also supports the following non-POSIX character classes: -# -# * /[[:word:]]/ - A character in one of the following Unicode -# general categories _Letter_, _Mark_, _Number_, -# Connector_Punctuation -# * /[[:ascii:]]/ - A character in the ASCII character set -# -# # U+06F2 is "EXTENDED ARABIC-INDIC DIGIT TWO" -# /[[:digit:]]/.match("\u06F2") #=> # -# /[[:upper:]][[:lower:]]/.match("Hello") #=> # -# /[[:xdigit:]][[:xdigit:]]/.match("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 quantifiers. -# -# * * - Zero or more times -# * + - One or more times -# * ? - Zero or one times (optional) -# * {n} - Exactly n times -# * {n,} - n or more times -# * {,m} - m or less times -# * {n,m} - At least n and -# at most m 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/) #=> # -# -# Repetition is greedy by default: as many occurrences as possible -# are matched while still allowing the overall match to succeed. By -# contrast, lazy matching makes the minimal amount of matches -# necessary for overall success. A greedy metacharacter can be made lazy by -# following it with ?. -# -# # Both patterns below match the string. The fist uses a greedy -# # quantifier so '.+' matches ''; the second uses a lazy -# # quantifier so '.+?' matches ''. -# /<.+>/.match("") #=> #"> -# /<.+?>/.match("") #=> #"> -# -# A quantifier followed by + matches possessively: 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 capturing. The text enclosed by the -# nth group of parentheses can be subsequently referred to -# with n. Within a pattern use the backreference -# \n; outside of the pattern use -# MatchData[n]. -# -# # '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") -# #=> # -# # 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 -# (?<name>) or (?'name') -# constructs. -# -# /\$(?\d+)\.(?\d+)/.match("$3.67") -# => # -# /\$(?\d+)\.(?\d+)/.match("$3.67")[:dollars] #=> "3" -# -# Named groups can be backreferenced with \k<name>, -# where _name_ is the group name. -# -# /(?[aeiou]).\k.\k/.match('ototomy') -# #=> # -# -# *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 =~ operator, the captured text is -# also assigned to local variables with corresponding names. -# -# /\$(?\d+)\.(?\d+)/ =~ "$3.67" #=> 0 -# dollars #=> "3" -# -# == Grouping -# -# Parentheses also group the terms they enclose, allowing them to be -# quantified as one atomic whole. -# -# # The pattern below matches a vowel followed by 2 word characters: -# # 'aen' -# /[aeiou]\w{2}/.match("Caenorhabditis elegans") #=> # -# # Whereas the following pattern matches a vowel followed by a word -# # character, twice, i.e. [aeiou]\w[aeiou]\w: 'enor'. -# /([aeiou]\w){2}/.match("Caenorhabditis elegans") -# #=> # -# -# The (?:...) 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") -# #=> # -# # 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") -# #=> # -# -# === Atomic Grouping -# -# Grouping can be made atomic with -# (?>pat). This causes the subexpression pat -# 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 pat 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 " in the pattern below matches the first character of -# # the string, then .* matches Quote". This causes the -# # overall match to fail, so the text matched by .* is -# # backtracked by one position, which leaves the final character of the -# # string available to match " -# /".*"/.match('"Quote"') #=> # -# # If .* is grouped atomically, it refuses to backtrack -# # Quote", even though this means that the overall match fails -# /"(?>.*)"/.match('"Quote"') #=> nil -# -# == Subexpression Calls -# -# The \g<name> 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 ( character and assigns it to the paren -# # group, tries to call that the paren sub-expression again -# # but fails, then matches a literal ). -# /\A(?\(\g*\))*\z/ =~ '()' -# -# -# /\A(?\(\g*\))*\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 paren -# 3. Matches a literal (, the first character in the string -# 4. Calls the paren group again, i.e. recurses back to the -# second step -# 5. Re-enters the paren group -# 6. Matches a literal (, the second character in the -# string -# 7. Try to call paren a third time, but fail because -# doing so would prevent an overall successful match -# 8. Match a literal ), the third character in the string. -# Marks the end of the second recursive call -# 9. Match a literal ), the fourth character in the string -# 10. Match the end of the string -# -# == Alternation -# -# The vertical bar metacharacter (|) combines two expressions into -# a single one that matches either of the expressions. Each expression is an -# alternative. -# -# /\w(and|or)\w/.match("Feliformia") #=> # -# /\w(and|or)\w/.match("furandi") #=> # -# /\w(and|or)\w/.match("dissemblance") #=> nil -# -# == Character Properties -# -# The \p{} construct matches characters with the named property, -# much like POSIX bracket classes. -# -# * /\p{Alnum}/ - Alphabetic and numeric character -# * /\p{Alpha}/ - Alphabetic character -# * /\p{Blank}/ - Space or tab -# * /\p{Cntrl}/ - Control character -# * /\p{Digit}/ - Digit -# * /\p{Graph}/ - Non-blank character (excludes spaces, control -# characters, and similar) -# * /\p{Lower}/ - Lowercase alphabetical character -# * /\p{Print}/ - Like \p{Graph}, but includes the space character -# * /\p{Punct}/ - Punctuation character -# * /\p{Space}/ - Whitespace character ([:blank:], newline, -# carriage return, etc.) -# * /\p{Upper}/ - Uppercase alphabetical -# * /\p{XDigit}/ - Digit allowed in a hexadecimal number (i.e., 0-9a-fA-F) -# * /\p{Word}/ - A member of one of the following Unicode general -# category Letter, Mark, Number, -# Connector\_Punctuation -# * /\p{ASCII}/ - A character in the ASCII character set -# * /\p{Any}/ - Any Unicode character (including unassigned -# characters) -# * /\p{Assigned}/ - An assigned character -# -# A Unicode character's General Category value can also be matched -# with \p{Ab} where Ab is the category's -# abbreviation as described below: -# -# * /\p{L}/ - 'Letter' -# * /\p{Ll}/ - 'Letter: Lowercase' -# * /\p{Lm}/ - 'Letter: Mark' -# * /\p{Lo}/ - 'Letter: Other' -# * /\p{Lt}/ - 'Letter: Titlecase' -# * /\p{Lu}/ - 'Letter: Uppercase -# * /\p{Lo}/ - 'Letter: Other' -# * /\p{M}/ - 'Mark' -# * /\p{Mn}/ - 'Mark: Nonspacing' -# * /\p{Mc}/ - 'Mark: Spacing Combining' -# * /\p{Me}/ - 'Mark: Enclosing' -# * /\p{N}/ - 'Number' -# * /\p{Nd}/ - 'Number: Decimal Digit' -# * /\p{Nl}/ - 'Number: Letter' -# * /\p{No}/ - 'Number: Other' -# * /\p{P}/ - 'Punctuation' -# * /\p{Pc}/ - 'Punctuation: Connector' -# * /\p{Pd}/ - 'Punctuation: Dash' -# * /\p{Ps}/ - 'Punctuation: Open' -# * /\p{Pe}/ - 'Punctuation: Close' -# * /\p{Pi}/ - 'Punctuation: Initial Quote' -# * /\p{Pf}/ - 'Punctuation: Final Quote' -# * /\p{Po}/ - 'Punctuation: Other' -# * /\p{S}/ - 'Symbol' -# * /\p{Sm}/ - 'Symbol: Math' -# * /\p{Sc}/ - 'Symbol: Currency' -# * /\p{Sc}/ - 'Symbol: Currency' -# * /\p{Sk}/ - 'Symbol: Modifier' -# * /\p{So}/ - 'Symbol: Other' -# * /\p{Z}/ - 'Separator' -# * /\p{Zs}/ - 'Separator: Space' -# * /\p{Zl}/ - 'Separator: Line' -# * /\p{Zp}/ - 'Separator: Paragraph' -# * /\p{C}/ - 'Other' -# * /\p{Cc}/ - 'Other: Control' -# * /\p{Cf}/ - 'Other: Format' -# * /\p{Cn}/ - 'Other: Not Assigned' -# * /\p{Co}/ - 'Other: Private Use' -# * /\p{Cs}/ - 'Other: Surrogate' -# -# Lastly, \p{} matches a character's Unicode script. The -# following scripts are supported: Arabic, Armenian, -# Balinese, Bengali, Bopomofo, Braille, -# Buginese, Buhid, Canadian_Aboriginal, Carian, -# Cham, Cherokee, Common, Coptic, -# Cuneiform, Cypriot, Cyrillic, Deseret, -# Devanagari, Ethiopic, Georgian, Glagolitic, -# Gothic, Greek, Gujarati, Gurmukhi, Han, -# Hangul, Hanunoo, Hebrew, Hiragana, -# Inherited, Kannada, Katakana, Kayah_Li, -# Kharoshthi, Khmer, Lao, Latin, Lepcha, -# Limbu, Linear_B, Lycian, Lydian, -# Malayalam, Mongolian, Myanmar, New_Tai_Lue, -# Nko, Ogham, Ol_Chiki, Old_Italic, -# Old_Persian, Oriya, Osmanya, Phags_Pa, -# Phoenician, Rejang, Runic, Saurashtra, -# Shavian, Sinhala, Sundanese, Syloti_Nagri, -# Syriac, Tagalog, Tagbanwa, Tai_Le, -# Tamil, Telugu, Thaana, Thai, Tibetan, -# Tifinagh, Ugaritic, Vai, and Yi. -# -# # Unicode codepoint U+06E9 is named "ARABIC PLACE OF SAJDAH" and -# # belongs to the Arabic script. -# /\p{Arabic}/.match("\u06E9") #=> # -# -# All character properties can be inverted by prefixing their name with a -# caret (^). -# -# # Letter 'A' is not in the Unicode Ll (Letter; Lowercase) category, so -# # this match succeeds -# /\p{^Ll}/.match("A") #=> # -# -# == Anchors -# -# Anchors are metacharacter that match the zero-width positions between -# characters, anchoring the match to a specific position. -# -# * ^ - Matches beginning of line -# * $ - Matches end of line -# * \A - Matches beginning of string. -# * \Z - Matches end of string. If string ends with a newline, -# it matches just before newline -# * \z - Matches end of string -# * \G - Matches point where last match finished -# * \b - Matches word boundaries when outside brackets; backspace -# (0x08) inside brackets -# * \B - Matches non-word boundaries -# * (?=pat) - Positive lookahead assertion: -# ensures that the following characters match pat, but doesn't -# include those characters in the matched text -# * (?!pat) - Negative lookahead assertion: -# ensures that the following characters do not match pat, but -# doesn't include those characters in the matched text -# * (?<=pat) - Positive lookbehind -# assertion: ensures that the preceding characters match pat, but -# doesn't include those characters in the matched text -# * (?pat) - Negative lookbehind -# assertion: ensures that the preceding characters do not match -# pat, 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") #=> # -# # 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") #=> # -# # The pattern below uses positive lookahead and positive lookbehind to -# # match text appearing in tags without including the tags in the -# # match -# /(?<=)\w+(?=<\/b>)/.match("Fortune favours the 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. -# -# * /pat/i - Ignore case -# * /pat/m - Treat a newline as a character matched by . -# * /pat/x - Ignore whitespace and comments in the pattern -# * /pat/o - Perform #{} interpolation only once -# -# i, m, and x can also be applied on the -# subexpression level with the -# (?on-off) construct, which -# enables options on, and disables options off for the -# expression enclosed by the parentheses. -# -# /a(?i:b)c/.match('aBc') #=> # -# /a(?i:b)c/.match('abc') #=> # -# -# == Free-Spacing Mode and Comments -# -# As mentioned above, the x option enables free-spacing -# mode. Literal white space inside the pattern is ignored, and the -# octothorpe (#) 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') #=> # -# -# *Note*: To match whitespace in an x pattern use an escape such as -# \s or \p{Space}. -# -# Comments can be included in a non-x pattern with the -# (?#comment) construct, where comment 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. -# -# * /pat/u - UTF-8 -# * /pat/e - EUC-JP -# * /pat/s - Windows-31J -# * /pat/n - 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 -# Encoding::CompatibilityError exception is raised. -# -# The Regexp#fixed_encoding? predicate indicates whether the regexp -# has a fixed encoding, that is one incompatible with ASCII. A -# regexp's encoding can be explicitly fixed by supplying -# Regexp::FIXEDENCODING as the second argument of -# Regexp.new: -# -# 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 as, a d, 4 as, and a -# c. -# -# 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 + and an enclosing * with nothing to -# differentiate which is in control of any particular character. The -# nondeterminism that results produces super-linear performance. (Consult -# Mastering Regular Expressions (3rd ed.), pp 222, by -# Jeffery Friedl, 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 as against a pattern of 29 optional -# # as followed by 29 mandatory as. -# Regexp.new('a?' * 29 + 'a' * 29) =~ 'a' * 29 -# -# The 29 optional as match the string, but this prevents the 29 -# mandatory as 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) -# #=> # -# -# +# -*- mode: rdoc; coding: utf-8; fill-column: 74; -*- +=begin rdoc + +Regular expressions (regexps) 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 /pat/ and +%r{pat} literals or the Regexp.new +constructor. + +A regexp is usually delimited with forward slashes (/). For +example: + + /hay/ =~ 'haystack' #=> 0 + /y/.match('haystack') #=> # + +If a string contains the pattern it is said to match. 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') #=> # + +Specifically, /st/ requires that the string contains the letter +_s_ followed by the letter _t_, so it matches _haystack_, also. + +== Metacharacters and Escapes + +The following are metacharacters (, ), +[, ], {, }, ., ?, ++, *. 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: \\\\\\. + + /1 \+ 2 = 3\?/.match('Does 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") + #=> # + +Arbitrary Ruby expressions can be embedded into patterns with the +#{...} construct. + + place = "東京都" + /#{place}/.match("Go to 東京都") + #=> # + +== Character Classes + +A character class is delimited with square brackets ([, +]) and lists characters that may appear at that point in the +match. /[ab]/ means _a_ or _b_, as opposed to /ab/ which +means _a_ followed by _b_. + + /W[aeiou]rd/.match("Word") #=> # + +Within a character class the hyphen (-) is a metacharacter +denoting an inclusive range of characters. [abcd] is equivalent +to [a-d]. A range can be followed by another range, so +[abcdwxyz] is equivalent to [a-dw-z]. The order in which +ranges or individual characters appear inside a character class is +irrelevant. + + /[0-9a-f]/.match('9f') #=> # + /[9f]/.match('9f') #=> # + +If the first character of a character class is a caret (^) the +class is inverted: it matches any character _except_ those named. + + /[^a-eg-z]/.match('f') #=> # + +A character class may contain another character class. By itself this +isn't useful because [a-z[0-9]] describes the same set as +[a-z0-9]. However, character classes also support the && +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: + +* /./ - Any character except a newline. +* /./m - Any character (the +m+ modifier enables multiline mode) +* /\w/ - A word character ([a-zA-Z0-9_]) +* /\W/ - A non-word character ([^a-zA-Z0-9_]) +* /\d/ - A digit character ([0-9]) +* /\D/ - A non-digit character ([^0-9]) +* /\h/ - A hexdigit character ([0-9a-fA-F]) +* /\H/ - A non-hexdigit character ([^0-9a-fA-F]) +* /\s/ - A whitespace character: /[ \t\r\n\f]/ +* /\S/ - A non-whitespace character: /[^ \t\r\n\f]/ + +POSIX bracket expressions 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, /\d/ +matches only the ASCII decimal digits (0-9); whereas /[[:digit:]]/ +matches any character in the Unicode _Nd_ category. + +* /[[:alnum:]]/ - Alphabetic and numeric character +* /[[:alpha:]]/ - Alphabetic character +* /[[:blank:]]/ - Space or tab +* /[[:cntrl:]]/ - Control character +* /[[:digit:]]/ - Digit +* /[[:graph:]]/ - Non-blank character (excludes spaces, control + characters, and similar) +* /[[:lower:]]/ - Lowercase alphabetical character +* /[[:print:]]/ - Like [:graph:], but includes the space character +* /[[:punct:]]/ - Punctuation character +* /[[:space:]]/ - Whitespace character ([:blank:], newline, + carriage return, etc.) +* /[[:upper:]]/ - Uppercase alphabetical +* /[[:xdigit:]]/ - Digit allowed in a hexadecimal number (i.e., + 0-9a-fA-F) + +Ruby also supports the following non-POSIX character classes: + +* /[[:word:]]/ - A character in one of the following Unicode + general categories _Letter_, _Mark_, _Number_, + Connector_Punctuation +* /[[:ascii:]]/ - A character in the ASCII character set + + # U+06F2 is "EXTENDED ARABIC-INDIC DIGIT TWO" + /[[:digit:]]/.match("\u06F2") #=> # + /[[:upper:]][[:lower:]]/.match("Hello") #=> # + /[[:xdigit:]][[:xdigit:]]/.match("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 quantifiers. + +* * - Zero or more times +* + - One or more times +* ? - Zero or one times (optional) +* {n} - Exactly n times +* {n,} - n or more times +* {,m} - m or less times +* {n,m} - At least n and + at most m 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/) #=> # + +Repetition is greedy by default: as many occurrences as possible +are matched while still allowing the overall match to succeed. By +contrast, lazy matching makes the minimal amount of matches +necessary for overall success. A greedy metacharacter can be made lazy by +following it with ?. + + # Both patterns below match the string. The fist uses a greedy + # quantifier so '.+' matches ''; the second uses a lazy + # quantifier so '.+?' matches ''. + /<.+>/.match("") #=> #"> + /<.+?>/.match("") #=> #"> + +A quantifier followed by + matches possessively: 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 capturing. The text enclosed by the +nth group of parentheses can be subsequently referred to +with n. Within a pattern use the backreference +\n; outside of the pattern use +MatchData[n]. + + # '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") + #=> # + # 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 +(?<name>) or (?'name') +constructs. + + /\$(?\d+)\.(?\d+)/.match("$3.67") + => # + /\$(?\d+)\.(?\d+)/.match("$3.67")[:dollars] #=> "3" + +Named groups can be backreferenced with \k<name>, +where _name_ is the group name. + + /(?[aeiou]).\k.\k/.match('ototomy') + #=> # + +*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 =~ operator, the captured text is +also assigned to local variables with corresponding names. + + /\$(?\d+)\.(?\d+)/ =~ "$3.67" #=> 0 + dollars #=> "3" + +== Grouping + +Parentheses also group the terms they enclose, allowing them to be +quantified as one atomic whole. + + # The pattern below matches a vowel followed by 2 word characters: + # 'aen' + /[aeiou]\w{2}/.match("Caenorhabditis elegans") #=> # + # Whereas the following pattern matches a vowel followed by a word + # character, twice, i.e. [aeiou]\w[aeiou]\w: 'enor'. + /([aeiou]\w){2}/.match("Caenorhabditis elegans") + #=> # + +The (?:...) 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") + #=> # + # 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") + #=> # + +=== Atomic Grouping + +Grouping can be made atomic with +(?>pat). This causes the subexpression pat +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 pat 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 " in the pattern below matches the first character of + # the string, then .* matches Quote". This causes the + # overall match to fail, so the text matched by .* is + # backtracked by one position, which leaves the final character of the + # string available to match " + /".*"/.match('"Quote"') #=> # + # If .* is grouped atomically, it refuses to backtrack + # Quote", even though this means that the overall match fails + /"(?>.*)"/.match('"Quote"') #=> nil + +== Subexpression Calls + +The \g<name> 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 ( character and assigns it to the paren + # group, tries to call that the paren sub-expression again + # but fails, then matches a literal ). + /\A(?\(\g*\))*\z/ =~ '()' + + + /\A(?\(\g*\))*\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 paren +3. Matches a literal (, the first character in the string +4. Calls the paren group again, i.e. recurses back to the + second step +5. Re-enters the paren group +6. Matches a literal (, the second character in the + string +7. Try to call paren a third time, but fail because + doing so would prevent an overall successful match +8. Match a literal ), the third character in the string. + Marks the end of the second recursive call +9. Match a literal ), the fourth character in the string +10. Match the end of the string + +== Alternation + +The vertical bar metacharacter (|) combines two expressions into +a single one that matches either of the expressions. Each expression is an +alternative. + + /\w(and|or)\w/.match("Feliformia") #=> # + /\w(and|or)\w/.match("furandi") #=> # + /\w(and|or)\w/.match("dissemblance") #=> nil + +== Character Properties + +The \p{} construct matches characters with the named property, +much like POSIX bracket classes. + +* /\p{Alnum}/ - Alphabetic and numeric character +* /\p{Alpha}/ - Alphabetic character +* /\p{Blank}/ - Space or tab +* /\p{Cntrl}/ - Control character +* /\p{Digit}/ - Digit +* /\p{Graph}/ - Non-blank character (excludes spaces, control + characters, and similar) +* /\p{Lower}/ - Lowercase alphabetical character +* /\p{Print}/ - Like \p{Graph}, but includes the space character +* /\p{Punct}/ - Punctuation character +* /\p{Space}/ - Whitespace character ([:blank:], newline, + carriage return, etc.) +* /\p{Upper}/ - Uppercase alphabetical +* /\p{XDigit}/ - Digit allowed in a hexadecimal number (i.e., 0-9a-fA-F) +* /\p{Word}/ - A member of one of the following Unicode general + category Letter, Mark, Number, + Connector\_Punctuation +* /\p{ASCII}/ - A character in the ASCII character set +* /\p{Any}/ - Any Unicode character (including unassigned + characters) +* /\p{Assigned}/ - An assigned character + +A Unicode character's General Category value can also be matched +with \p{Ab} where Ab is the category's +abbreviation as described below: + +* /\p{L}/ - 'Letter' +* /\p{Ll}/ - 'Letter: Lowercase' +* /\p{Lm}/ - 'Letter: Mark' +* /\p{Lo}/ - 'Letter: Other' +* /\p{Lt}/ - 'Letter: Titlecase' +* /\p{Lu}/ - 'Letter: Uppercase +* /\p{Lo}/ - 'Letter: Other' +* /\p{M}/ - 'Mark' +* /\p{Mn}/ - 'Mark: Nonspacing' +* /\p{Mc}/ - 'Mark: Spacing Combining' +* /\p{Me}/ - 'Mark: Enclosing' +* /\p{N}/ - 'Number' +* /\p{Nd}/ - 'Number: Decimal Digit' +* /\p{Nl}/ - 'Number: Letter' +* /\p{No}/ - 'Number: Other' +* /\p{P}/ - 'Punctuation' +* /\p{Pc}/ - 'Punctuation: Connector' +* /\p{Pd}/ - 'Punctuation: Dash' +* /\p{Ps}/ - 'Punctuation: Open' +* /\p{Pe}/ - 'Punctuation: Close' +* /\p{Pi}/ - 'Punctuation: Initial Quote' +* /\p{Pf}/ - 'Punctuation: Final Quote' +* /\p{Po}/ - 'Punctuation: Other' +* /\p{S}/ - 'Symbol' +* /\p{Sm}/ - 'Symbol: Math' +* /\p{Sc}/ - 'Symbol: Currency' +* /\p{Sc}/ - 'Symbol: Currency' +* /\p{Sk}/ - 'Symbol: Modifier' +* /\p{So}/ - 'Symbol: Other' +* /\p{Z}/ - 'Separator' +* /\p{Zs}/ - 'Separator: Space' +* /\p{Zl}/ - 'Separator: Line' +* /\p{Zp}/ - 'Separator: Paragraph' +* /\p{C}/ - 'Other' +* /\p{Cc}/ - 'Other: Control' +* /\p{Cf}/ - 'Other: Format' +* /\p{Cn}/ - 'Other: Not Assigned' +* /\p{Co}/ - 'Other: Private Use' +* /\p{Cs}/ - 'Other: Surrogate' + +Lastly, \p{} matches a character's Unicode script. The +following scripts are supported: Arabic, Armenian, +Balinese, Bengali, Bopomofo, Braille, +Buginese, Buhid, Canadian_Aboriginal, Carian, +Cham, Cherokee, Common, Coptic, +Cuneiform, Cypriot, Cyrillic, Deseret, +Devanagari, Ethiopic, Georgian, Glagolitic, +Gothic, Greek, Gujarati, Gurmukhi, Han, +Hangul, Hanunoo, Hebrew, Hiragana, +Inherited, Kannada, Katakana, Kayah_Li, +Kharoshthi, Khmer, Lao, Latin, Lepcha, +Limbu, Linear_B, Lycian, Lydian, +Malayalam, Mongolian, Myanmar, New_Tai_Lue, +Nko, Ogham, Ol_Chiki, Old_Italic, +Old_Persian, Oriya, Osmanya, Phags_Pa, +Phoenician, Rejang, Runic, Saurashtra, +Shavian, Sinhala, Sundanese, Syloti_Nagri, +Syriac, Tagalog, Tagbanwa, Tai_Le, +Tamil, Telugu, Thaana, Thai, Tibetan, +Tifinagh, Ugaritic, Vai, and Yi. + + # Unicode codepoint U+06E9 is named "ARABIC PLACE OF SAJDAH" and + # belongs to the Arabic script. + /\p{Arabic}/.match("\u06E9") #=> # + +All character properties can be inverted by prefixing their name with a +caret (^). + + # Letter 'A' is not in the Unicode Ll (Letter; Lowercase) category, so + # this match succeeds + /\p{^Ll}/.match("A") #=> # + +== Anchors + +Anchors are metacharacter that match the zero-width positions between +characters, anchoring the match to a specific position. + +* ^ - Matches beginning of line +* $ - Matches end of line +* \A - Matches beginning of string. +* \Z - Matches end of string. If string ends with a newline, + it matches just before newline +* \z - Matches end of string +* \G - Matches point where last match finished +* \b - Matches word boundaries when outside brackets; backspace + (0x08) inside brackets +* \B - Matches non-word boundaries +* (?=pat) - Positive lookahead assertion: + ensures that the following characters match pat, but doesn't + include those characters in the matched text +* (?!pat) - Negative lookahead assertion: + ensures that the following characters do not match pat, but + doesn't include those characters in the matched text +* (?<=pat) - Positive lookbehind + assertion: ensures that the preceding characters match pat, but + doesn't include those characters in the matched text +* (?pat) - Negative lookbehind + assertion: ensures that the preceding characters do not match + pat, 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") #=> # + # 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") #=> # + # The pattern below uses positive lookahead and positive lookbehind to + # match text appearing in tags without including the tags in the + # match + /(?<=)\w+(?=<\/b>)/.match("Fortune favours the 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. + +* /pat/i - Ignore case +* /pat/m - Treat a newline as a character matched by . +* /pat/x - Ignore whitespace and comments in the pattern +* /pat/o - Perform #{} interpolation only once + +i, m, and x can also be applied on the +subexpression level with the +(?on-off) construct, which +enables options on, and disables options off for the +expression enclosed by the parentheses. + + /a(?i:b)c/.match('aBc') #=> # + /a(?i:b)c/.match('abc') #=> # + +== Free-Spacing Mode and Comments + +As mentioned above, the x option enables free-spacing +mode. Literal white space inside the pattern is ignored, and the +octothorpe (#) 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') #=> # + +*Note*: To match whitespace in an x pattern use an escape such as +\s or \p{Space}. + +Comments can be included in a non-x pattern with the +(?#comment) construct, where comment 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. + +* /pat/u - UTF-8 +* /pat/e - EUC-JP +* /pat/s - Windows-31J +* /pat/n - 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 +Encoding::CompatibilityError exception is raised. + +The Regexp#fixed_encoding? predicate indicates whether the regexp +has a fixed encoding, that is one incompatible with ASCII. A +regexp's encoding can be explicitly fixed by supplying +Regexp::FIXEDENCODING as the second argument of +Regexp.new: + + 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 as, a d, 4 as, and a +c. + + 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 + and an enclosing * with nothing to +differentiate which is in control of any particular character. The +nondeterminism that results produces super-linear performance. (Consult +Mastering Regular Expressions (3rd ed.), pp 222, by +Jeffery Friedl, 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 as against a pattern of 29 optional + # as followed by 29 mandatory as. + Regexp.new('a?' * 29 + 'a' * 29) =~ 'a' * 29 + +The 29 optional as match the string, but this prevents the 29 +mandatory as 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) + #=> # +=end class Regexp; end