A Regex
represents a regular expression, a pattern that describes the contents of strings. A Regex
can determine whether or not a string matches its description, and extract the parts of the string that match.
A Regex
can be created using the literal syntax, in which it is delimited by forward slashes (/
):
/hay/ =~ "haystack" # => 0 /y/.match("haystack") # => Regex::MatchData("y")
Interpolation works in regular expression literals just as it does in string literals. Be aware that using this feature will cause an exception to be raised at runtime, if the resulting string would not be a valid regular expression.
x = "a" /#{x}/.match("asdf") # => Regex::MatchData("a") x = "(" /#{x}/ # raises ArgumentError
When we check to see if a particular regular expression describes a string, we can say that we are performing a match or matching one against the other. If we find that a regular expression does describe a string, we say that it matches, and we can refer to a part of the string that was described as a match.
Here "haystack"
does not contain the pattern /needle/
, so it doesn't match:
/needle/.match("haystack") # => nil
Here "haystack"
contains the pattern /hay/
, so it matches:
/hay/.match("haystack") # => Regex::MatchData("hay")
Regex methods that perform a match usually return a truthy value if there was a match and nil
if there was no match. After performing a match, the special variable $~
will be an instance of Regex::MatchData
if it matched, nil
otherwise.
When matching a regular expression using #=~
(either String#=~
or Regex#=~
), the returned value will be the index of the first match in the string if the expression matched, nil
otherwise.
/stack/ =~ "haystack" # => 3 "haystack" =~ /stack/ # => 3 $~ # => Regex::MatchData("stack") /needle/ =~ "haystack" # => nil "haystack" =~ /needle/ # => nil $~ # raises Exception
When matching a regular expression using #match
(either String#match
or Regex#match
), the returned value will be a Regex::MatchData
if the expression matched, nil
otherwise.
/hay/.match("haystack") # => Regex::MatchData("hay") "haystack".match(/hay/) # => Regex::MatchData("hay") $~ # => Regex::MatchData("hay") /needle/.match("haystack") # => nil "haystack".match(/needle/) # => nil $~ # raises Exception
Regular expressions have their own language for describing strings.
Many programming languages and tools implement their own regular expression language, but Crystal uses PCRE, a popular C library for providing regular expressions. Here give a brief summary of the most basic features of regular expressions - grouping, repetition, and alternation - but the feature set of PCRE extends far beyond these, and we don't attempt to describe it in full here. For more information, refer to the PCRE documentation, especially the full pattern syntax or syntax quick reference.
The regular expression language can be used to match much more than just the static substrings in the above examples. Certain characters, called metacharacters, are given special treatment in regular expressions, and can be used to describe more complex patterns. To match metacharacters literally in a regular expression, they must be escaped by being preceded with a backslash (\
). .escape
will do this automatically for a given String.
A group of characters (often called a capture group or subpattern) can be identified by enclosing it in parentheses (()
). The contents of each capture group can be extracted on a successful match:
/a(sd)f/.match("_asdf_") # => Regex::MatchData("asdf" 1:"sd") /a(sd)f/.match("_asdf_").try &.[1] # => "sd" /a(?<grp>sd)f/.match("_asdf_") # => Regex::MatchData("asdf" grp:"sd") /a(?<grp>sd)f/.match("_asdf_").try &.["grp"] # => "sd"
Capture groups are indexed starting from 1. Methods that accept a capture group index will usually also accept 0 to refer to the full match. Capture groups can also be given names, using the (?<name>...)
syntax, as in the previous example.
Following a match, the special variables $N (e.g., $1, $2, $3, ...) can be used to access a capture group. Trying to access an invalid capture group will raise an exception. Note that it is possible to have a successful match with a nil capture:
/(spice)(s)?/.match("spice") # => Regex::MatchData("spice" 1:"spice" 2:nil) 1 # => "spice" 2 # => raises Exception
This can be mitigated by using the nilable version of the above: $N?, (e.g., $1? $2?, $3?, ...). Changing the above to use $2?
instead of $2
would return nil
. $2?.nil?
would return true
.
A character or group can be repeated or made optional using an asterisk (*
- zero or more), a plus sign (#+
- one or more), integer bounds in curly braces ({n,m}
) (at least n
, no more than m
), or a question mark (?
) (zero or one).
/fo*/.match("_f_") # => Regex::MatchData("f") /fo+/.match("_f_") # => nil /fo*/.match("_foo_") # => Regex::MatchData("foo") /fo{3,}/.match("_foo_") # => nil /fo{1,3}/.match("_foo_") # => Regex::MatchData("foo") /fo*/.match("_foo_") # => Regex::MatchData("foo") /fo*/.match("_foooooooo_") # => Regex::MatchData("foooooooo") /fo{,3}/.match("_foooo_") # => nil /f(op)*/.match("fopopo") # => Regex::MatchData("fopop" 1:"op") /foo?bar/.match("foobar") # => Regex::MatchData("foobar") /foo?bar/.match("fobar") # => Regex::MatchData("fobar")
Alternatives can be separated using a vertical bar (|
). Any single character can be represented by dot (.
). When matching only one character, specific alternatives can be expressed as a character class, enclosed in square brackets ([]
):
/foo|bar/.match("foo") # => Regex::MatchData("foo") /foo|bar/.match("bar") # => Regex::MatchData("bar") /_(x|y)_/.match("_x_") # => Regex::MatchData("_x_" 1:"x") /_(x|y)_/.match("_y_") # => Regex::MatchData("_y_" 1:"y") /_(x|y)_/.match("_(x|y)_") # => nil /_(x|y)_/.match("_(x|y)_") # => nil /_._/.match("_x_") # => Regex::MatchData("_x_") /_[xyz]_/.match("_x_") # => Regex::MatchData("_x_") /_[a-z]_/.match("_x_") # => Regex::MatchData("_x_") /_[^a-z]_/.match("_x_") # => nil /_[^a-wy-z]_/.match("_x_") # => Regex::MatchData("_x_")
Regular expressions can be defined with these 3 optional flags:
i
: ignore case (PCRE_CASELESS)m
: multiline (PCRE_MULTILINE and PCRE_DOTALL)x
: extended (PCRE_EXTENDED)/asdf/ =~ "ASDF" # => nil /asdf/i =~ "ASDF" # => 0 /^z/i =~ "ASDF\nZ" # => nil /^z/im =~ "ASDF\nZ" # => 5
PCRE supports other encodings, but Crystal strings are UTF-8 only, so Crystal regular expressions are also UTF-8 only (by default).
PCRE optionally permits named capture groups (named subpatterns) to not be unique. Crystal exposes the name table of a Regex
as a Hash
of String
=> Int32
, and therefore requires named capture groups to have unique names within a single Regex
.
{' ', '.', '\\', '+', '*', '?', '[', '^', ']', '$', '(', ')', '{', '}', '=', '!', '<', '>', '|', ':', '-'}
List of metacharacters that need to be escaped.
See Regex.needs_escape?
and Regex.escape
.
Union.
Union.
Determines Regex's source validity.
Returns a String
constructed by escaping any metacharacters in str.
Returns true
if char need to be escaped, false
otherwise.
Returns true
if str need to be escaped, false
otherwise.
Union.
Equality.
Case equality.
Match.
Match.
Returns a shallow copy of this object.
Convert to String
in literal format.
Match at character index.
Match at byte index.
Match at character index.
Match at byte index.
Returns a Hash
where the values are the names of capture groups and the keys are their indexes.
Returns a Regex::Options
representing the optional flags applied to this Regex
.
Convert to String
in subpattern format.
Reference
Reference
Object
Object
Union. Returns a Regex
that matches any of patterns.
All capture groups in the patterns after the first one will have their indexes offset.
re = Regex.union([/skiing/i, "sledding"]) re.match("Skiing") # => Regex::MatchData("Skiing") re.match("sledding") # => Regex::MatchData("sledding") re = Regex.union({/skiing/i, "sledding"}) re.match("Skiing") # => Regex::MatchData("Skiing") re.match("sledding") # => Regex::MatchData("sledding")
Union. Returns a Regex
that matches any of patterns.
All capture groups in the patterns after the first one will have their indexes offset.
re = Regex.union(/skiing/i, "sledding") re.match("Skiing") # => Regex::MatchData("Skiing") re.match("sledding") # => Regex::MatchData("sledding")
Determines Regex's source validity. If it is, nil
is returned. If it's not, a String
containing the error message is returned.
Regex.error?("(foo|bar)") # => nil Regex.error?("(foo|bar") # => "missing ) at 8"
Returns a String
constructed by escaping any metacharacters in str.
string = Regex.escape("*?{}.") # => "\\*\\?\\{\\}\\." /#{string}/ # => /\*\?\{\}\./
Returns true
if char need to be escaped, false
otherwise.
Regex.needs_escape?('*') # => true Regex.needs_escape?('@') # => false
Returns true
if str need to be escaped, false
otherwise.
Regex.needs_escape?("10$") # => true Regex.needs_escape?("foo") # => false
Union. Returns a Regex
that matches either of the operands.
All capture groups in the second operand will have their indexes offset.
re = /skiing/i + /sledding/ re.match("Skiing") # => Regex::MatchData("Skiing") re.match("sledding") # => Regex::MatchData("sledding")
Equality. Two regexes are equal if their sources and options are the same.
/abc/ == /abc/i # => false /abc/i == /ABC/i # => false /abc/i == /abc/i # => true
Case equality. This is equivalent to #match
or #=~
but only returns true
or false
. Used in case
expressions. The special variable $~
will contain a Regex::MatchData
if there was a match, nil
otherwise.
a = "HELLO" b = case a when /^[a-z]*$/ "Lower case" when /^[A-Z]*$/ "Upper case" else "Mixed case" end b # => "Upper case"
Match. When the argument is not a String
, always returns nil
.
/at/ =~ "input data" # => 7 /ax/ =~ "input data" # => nil
Match. Matches a regular expression against other and returns the starting position of the match if other is a matching String
, otherwise nil
. $~
will contain a Regex::MatchData
if there was a match, nil
otherwise.
/at/ =~ "input data" # => 7 /ax/ =~ "input data" # => nil
Returns a shallow copy of this object.
This allocates a new object and copies the contents of self
into it.
Match at character index. Matches a regular expression against String
str. Starts at the character index given by pos if given, otherwise at the start of str. Returns a Regex::MatchData
if str matched, otherwise nil
. $~
will contain the same value that was returned.
/(.)(.)(.)/.match("abc").try &.[2] # => "b" /(.)(.)/.match("abc", 1).try &.[2] # => "c" /(.)(.)/.match("クリスタル", 3).try &.[2] # => "ル"
Match at byte index. Matches a regular expression against String
str. Starts at the byte index given by pos if given, otherwise at the start of str. Returns a Regex::MatchData
if str matched, otherwise nil
. $~
will contain the same value that was returned.
/(.)(.)(.)/.match_at_byte_index("abc").try &.[2] # => "b" /(.)(.)/.match_at_byte_index("abc", 1).try &.[2] # => "c" /(.)(.)/.match_at_byte_index("クリスタル", 3).try &.[2] # => "ス"
Returns a Hash
where the values are the names of capture groups and the keys are their indexes. Non-named capture groups will not have entries in the Hash
. Capture groups are indexed starting from 1
.
/(.)/.name_table # => {} /(?<foo>.)/.name_table # => {1 => "foo"} /(?<foo>.)(?<bar>.)/.name_table # => {2 => "bar", 1 => "foo"} /(.)(?<foo>.)(.)(?<bar>.)(.)/.name_table # => {4 => "bar", 2 => "foo"}
Returns a Regex::Options
representing the optional flags applied to this Regex
.
/ab+c/ix.options # => Regex::Options::IGNORE_CASE | Regex::Options::EXTENDED /ab+c/ix.options.to_s # => "IGNORE_CASE | EXTENDED"
Convert to String
in subpattern format. Produces a String
which can be embedded in another Regex
via interpolation, where it will be interpreted as a non-capturing subexpression in another regular expression.
re = /A*/i # => /A*/i re.to_s # => "(?i-msx:A*)" "Crystal".match(/t#{re}l/) # => Regex::MatchData("tal") re = /A*/ # => "(?-imsx:A*)" "Crystal".match(/t#{re}l/) # => nil
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Licensed under the Apache License, Version 2.0.
https://crystal-lang.org/api/0.35.1/Regex.html