Regexp serialization/deserialization
A Regexp holds a regular expression, used to match a pattern
against strings. Regexps are created using the /.../ and
%r{...} literals, and by the Regexp::new
constructor.
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') #=> #<MatchData "y">
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') #=> #<MatchData "hay">
Specifically, /st/ requires that the string contains the
letter s followed by the letter t, so it matches
haystack, also.
=~ and #match
Pattern matching may be achieved by using =~ operator or #match method.
=~ operator
=~ is Ruby's basic pattern-matching operator. When one
operand is a regular expression and the other is a string then the regular
expression is used as a pattern to match against the string. (This
operator is equivalently defined by Regexp and
String so the order of String and Regexp do not
matter. Other classes may have different implementations of
=~.) If a match is found, the operator returns index of first
match in string, otherwise it returns nil.
/hay/ =~ 'haystack' #=> 0 'haystack' =~ /hay/ #=> 0 /a/ =~ 'haystack' #=> 1 /u/ =~ 'haystack' #=> nil
Using =~ operator with a String and Regexp the $~ global variable is set
after a successful match. $~ holds a MatchData object. ::last_match is equivalent to
$~.
#match method
match method return a MatchData object :
/st/.match('haystack') #=> #<MatchData "st">
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?') #=> #<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
#{...} construct.
place = "東京都"
/#{place}/.match("Go to 東京都")
#=> #<MatchData "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") #=> #<MatchData "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') #=> #<MatchData "9">
/[9f]/.match('9f') #=> #<MatchData "9">
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') #=> #<MatchData "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 (themmodifier 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") #=> #<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 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/) #=> #<MatchData "Hello">
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 first 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 + 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
n<sup>th</sup> 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")
#=> #<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
(?<name>) or
(?'name') 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
\k<name>, 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 =~ 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 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") #=> #<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 (?:…) 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 readability.
# 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 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 needlessly.
# 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 \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 <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
-
Matches at the beginning of the string, i.e. before the first character.
-
Enters a named capture group called
paren -
Matches a literal (, the first character in the string
-
Calls the
parengroup again, i.e. recurses back to the second step -
Re-enters the
parengroup -
Matches a literal (, the second character in the string
-
Try to call
parena third time, but fail because doing so would prevent an overall successful match -
Match a literal ), the third character in the string. Marks the end of the second recursive call
-
Match a literal ), the fourth character in the string
-
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") #=> #<MatchData "form" 1:"or">
/\w(and|or)\w/.match("furandi") #=> #<MatchData "randi" 1:"and">
/\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") #=> #<MatchData "\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") #=> #<MatchData "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) when 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") #=> #<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.
-
/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') #=> #<MatchData "aBc">
/a(?i:b)c/.match('abc') #=> #<MatchData "abc">
Options may also be used with Regexp.new:
Regexp.new("abc", Regexp::IGNORECASE) #=> /abc/i
Regexp.new("abc", Regexp::MULTILINE) #=> /abc/m
Regexp.new("abc # Comment", Regexp::EXTENDED) #=> /abc # Comment/x
Regexp.new("abc", Regexp::IGNORECASE | Regexp::MULTILINE) #=> /abc/mi
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') #=> #<MatchData "3.14" 1:".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)Special global variables
Pattern matching sets some global variables :
-
$~is equivalent to ::last_match; -
$&contains the complete matched text; -
$`contains string before match; -
$'contains string after match; -
$1,$2and so on contain text matching first, second, etc capture group; -
$+contains last capture group.
Example:
m = /s(\w{2}).*(c)/.match('haystack') #=> #<MatchData "stac" 1:"ta" 2:"c">
$~ #=> #<MatchData "stac" 1:"ta" 2:"c">
Regexp.latch_match #=> #<MatchData "stac" 1:"ta" 2:"c">
$& #=> "stac"
# same as m[0]
$` #=> "hay"
# same as m.pre_match
$' #=> "k"
# same as m.post_match
$1 #=> "ta"
# same as m[1]
$2 #=> "c"
# same as m[2]
$3 #=> nil
# no third group in pattern
$+ #=> "c"
# same as m[-1]
These global variables are thread-local and method-local variables.
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' #=> "aaaaaaaaaaaaaaaaaaaaaaaaadaaaac"
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 #=> 26
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 <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 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.
The best way to improve performance is to significantly reduce the amount of backtracking needed. For this case, instead of individually matching 29 optional as, a range of optional as can be matched all at once with a{0,29}:
Regexp.new('a{0,29}' + 'a' * 29) =~ 'a' * 29