Raised with the interrupt signal is received, typically because the user pressed on Control-C (on most posix platforms). As such, it is a subclass of SignalException.
begin puts "Press ctrl-C when you get bored" loop {} rescue Interrupt => e puts "Note: You will typically use Signal.trap instead." end
produces:
Press ctrl-C when you get bored
then waits until it is interrupted with Control-C and then prints:
Note: You will typically use Signal.trap instead.
The most standard error types are subclasses of StandardError. A rescue clause without an explicit Exception class will rescue all StandardErrors (and only those).
def foo raise "Oups" end foo rescue "Hello" #=> "Hello"
On the other hand:
require 'does/not/exist' rescue "Hi"
raises the exception:
LoadError: no such file to load -- does/not/exist
Raised when encountering an object that is not of the expected type.
[1, 2, 3].first("two")
raises the exception:
TypeError: no implicit conversion of String into Integer
Raised when the arguments are wrong and there isn’t a more specific Exception class.
Ex: passing the wrong number of arguments
[1, 2, 3].first(4, 5)
raises the exception:
ArgumentError: wrong number of arguments (given 2, expected 1)
Ex: passing an argument that is not acceptable:
[1, 2, 3].first(-4)
raises the exception:
ArgumentError: negative array size
Raised when the given index is invalid.
a = [:foo, :bar] a.fetch(0) #=> :foo a[4] #=> nil a.fetch(4) #=> IndexError: index 4 outside of array bounds: -2...2
Raised when the specified key is not found. It is a subclass of IndexError.
h = {"foo" => :bar} h.fetch("foo") #=> :bar h.fetch("baz") #=> KeyError: key not found: "baz"
ScriptError is the superclass for errors raised when a script can not be executed because of a LoadError, NotImplementedError or a SyntaxError. Note these type of ScriptErrors are not StandardError and will not be rescued unless it is specified explicitly (or its ancestor Exception).
Raised when encountering Ruby code with an invalid syntax.
eval("1+1=2")
raises the exception:
SyntaxError: (eval):1: syntax error, unexpected '=', expecting $end
Raised when a feature is not implemented on the current platform. For example, methods depending on the fsync or fork system calls may raise this exception if the underlying operating system or Ruby runtime does not support them.
Note that if fork raises a NotImplementedError, then respond_to?(:fork) returns false.
Raised when a given name is invalid or undefined.
puts foo
raises the exception:
NameError: undefined local variable or method `foo' for main:Object
Since constant names must start with a capital:
Integer.const_set :answer, 42
raises the exception:
NameError: wrong constant name answer
Raised when a method is called on a receiver which doesn’t have it defined and also fails to respond with method_missing.
"hello".to_ary
raises the exception:
NoMethodError: undefined method `to_ary' for "hello":String
A generic error class raised when an invalid operation is attempted.
[1, 2, 3].freeze << 4
raises the exception:
RuntimeError: can't modify frozen Array
Kernel.raise will raise a RuntimeError if no Exception class is specified.
raise "ouch"
raises the exception:
RuntimeError: ouch
Raised when attempting a potential unsafe operation, typically when the $SAFE level is raised above 0.
foo = "bar" proc = Proc.new do $SAFE = 3 foo.untaint end proc.call
raises the exception:
SecurityError: Insecure: Insecure operation `untaint' at level 3
Raised when memory allocation fails.
SystemCallError is the base class for all low-level platform-dependent errors.
The errors available on the current platform are subclasses of SystemCallError and are defined in the Errno module.
File.open("does/not/exist")
raises the exception:
Errno::ENOENT: No such file or directory - does/not/exist
A Range represents an interval—a set of values with a beginning and an end. Ranges may be constructed using the s..e and s...e literals, or with Range::new. Ranges constructed using .. run from the beginning to the end inclusively. Those created using ... exclude the end value. When used as an iterator, ranges return each value in the sequence.
(-1..-5).to_a #=> [] (-5..-1).to_a #=> [-5, -4, -3, -2, -1] ('a'..'e').to_a #=> ["a", "b", "c", "d", "e"] ('a'...'e').to_a #=> ["a", "b", "c", "d"]
Ranges can be constructed using any objects that can be compared using the <=> operator. Methods that treat the range as a sequence (each and methods inherited from Enumerable) expect the begin object to implement a succ method to return the next object in sequence. The step and include? methods require the begin object to implement succ or to be numeric.
In the Xs class below both <=> and succ are implemented so Xs can be used to construct ranges. Note that the Comparable module is included so the == method is defined in terms of <=>.
class Xs # represent a string of 'x's include Comparable attr :length def initialize(n) @length = n end def succ Xs.new(@length + 1) end def <=>(other) @length <=> other.length end def to_s sprintf "%2d #{inspect}", @length end def inspect 'x' * @length end end
An example of using Xs to construct a range:
r = Xs.new(3)..Xs.new(6) #=> xxx..xxxxxx r.to_a #=> [xxx, xxxx, xxxxx, xxxxxx] r.member?(Xs.new(5)) #=> true
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.
Here ‘haystack’ does not contain the pattern ‘needle’, so it doesn’t match:
/needle/.match('haystack') #=> nil
Here ‘haystack’ contains the pattern ‘hay’, so it matches:
/hay/.match('haystack') #=> #<MatchData "hay">
Specifically, /st/ requires that the string contains the letter s followed by the letter t, so it matches haystack, also.
=~ and Regexp#match Pattern matching may be achieved by using =~ operator or Regexp#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. Regexp.last_match is equivalent to $~.
Regexp#match method The match method returns a MatchData object:
/st/.match('haystack') #=> #<MatchData "st">
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">
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 (the m modifier enables multiline mode)
/\w/ - A word character ([a-zA-Z0-9_])
/\W/ - A non-word character ([^a-zA-Z0-9_]). Please take a look at Bug #4044 if using /\W/ with the /i modifier.
/\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\v]/
/\S/ - A non-whitespace character: /[^ \t\r\n\f\v]/
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">
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.
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"
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:
/[aeiou]\w{2}/.match("Caenorhabditis elegans") #=> #<MatchData "aen">
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") #=> #<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 first 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">
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 " 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"') #=> #<MatchData "\"Quote\"">
If .* is grouped atomically, it refuses to backtrack Quote“, even though this means that the overall match fails
/"(?>.*)"/.match('"Quote"') #=> nil
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.
This pattern 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(?<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 paren group again, i.e. recurses back to the second step
Re-enters the paren group
Matches a literal (, the second character in the string
Try to call paren a 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
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
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 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 first matching position:
In methods like String#gsub and String#scan, it changes on each iteration. It initially matches the beginning of subject, and in each following iteration it matches where the last match finished.
" a b c".gsub(/ /, '_') #=> "____a_b_c" " a b c".gsub(/\G /, '_') #=> "____a b c"
In methods like Regexp#match and String#match that take an (optional) offset, it matches where the search begins.
"hello, world".match(/,/, 3) #=> #<MatchData ","> "hello, world".match(/\G,/, 3) #=> nil
\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 tags without including the tags in the match:
/(?<=<b>)\w+(?=<\/b>)/.match("Fortune favours the <b>bold</b>") #=> #<MatchData "bold">
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
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 organized 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">
There are a number of strategies for matching whitespace:
Use a pattern such as \s or \p{Space}.
Use escaped whitespace such as \ , i.e. a space preceded by a backslash.
Use a character class such as [ ].
Comments can be included in a non-x pattern with the (?#comment) construct, where comment is arbitrary text ignored by the regexp engine.
Comments in regexp literals cannot include unescaped terminator characters.
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)
Pattern matching sets some global variables :
$~ is equivalent to Regexp.last_match;
$& contains the complete matched text;
$` contains string before match;
$' contains string after match;
$1, $2 and 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.last_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.
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 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.
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
Ripper is a Ruby script parser.
You can get information from the parser with event-based style. Information such as abstract syntax trees or simple lexical analysis of the Ruby program.
Ripper provides an easy interface for parsing your program into a symbolic expression tree (or S-expression).
Understanding the output of the parser may come as a challenge, it’s recommended you use PP to format the output for legibility.
require 'ripper'
require 'pp'
pp Ripper.sexp('def hello(world) "Hello, #{world}!"; end')
#=> [:program,
[[:def,
[:@ident, "hello", [1, 4]],
[:paren,
[:params, [[:@ident, "world", [1, 10]]], nil, nil, nil, nil, nil, nil]],
[:bodystmt,
[[:string_literal,
[:string_content,
[:@tstring_content, "Hello, ", [1, 18]],
[:string_embexpr, [[:var_ref, [:@ident, "world", [1, 27]]]]],
[:@tstring_content, "!", [1, 33]]]]],
nil,
nil,
nil]]]]
You can see in the example above, the expression starts with :program.
From here, a method definition at :def, followed by the method’s identifier :@ident. After the method’s identifier comes the parentheses :paren and the method parameters under :params.
Next is the method body, starting at :bodystmt (stmt meaning statement), which contains the full definition of the method.
In our case, we’re simply returning a String, so next we have the :string_literal expression.
Within our :string_literal you’ll notice two @tstring_content, this is the literal part for Hello, and !. Between the two @tstring_content statements is a :string_embexpr, where embexpr is an embedded expression. Our expression consists of a local variable, or var_ref, with the identifier (@ident) of world.
ruby 1.9 (support CVS HEAD only)
bison 1.28 or later (Other yaccs do not work)
Ruby License. Minero Aoki aamine@loveruby.net http://i.loveruby.net
SocketError is the error class for socket.
StringScanner provides for lexical scanning operations on a String. Here is an example of its usage:
s = StringScanner.new('This is an example string') s.eos? # -> false p s.scan(/\w+/) # -> "This" p s.scan(/\w+/) # -> nil p s.scan(/\s+/) # -> " " p s.scan(/\s+/) # -> nil p s.scan(/\w+/) # -> "is" s.eos? # -> false p s.scan(/\s+/) # -> " " p s.scan(/\w+/) # -> "an" p s.scan(/\s+/) # -> " " p s.scan(/\w+/) # -> "example" p s.scan(/\s+/) # -> " " p s.scan(/\w+/) # -> "string" s.eos? # -> true p s.scan(/\s+/) # -> nil p s.scan(/\w+/) # -> nil
Scanning a string means remembering the position of a scan pointer, which is just an index. The point of scanning is to move forward a bit at a time, so matches are sought after the scan pointer; usually immediately after it.
Given the string “test string”, here are the pertinent scan pointer positions:
t e s t s t r i n g
0 1 2 ... 1
0
When you scan for a pattern (a regular expression), the match must occur at the character after the scan pointer. If you use scan_until, then the match can occur anywhere after the scan pointer. In both cases, the scan pointer moves just beyond the last character of the match, ready to scan again from the next character onwards. This is demonstrated by the example above.
Method Categories There are other methods besides the plain scanners. You can look ahead in the string without actually scanning. You can access the most recent match. You can modify the string being scanned, reset or terminate the scanner, find out or change the position of the scan pointer, skip ahead, and so on.
beginning_of_line? (bol?)
Data There are aliases to several of the methods.
OLEProperty helper class of Property with arguments.