19.07.2009
Ruby Regular Expression API cheat sheet
Creating a regular expression
>> /world/mix
=> Regexp(/world/mix)
>> Regexp.new("world", Regexp::MULTILINE | Regexp::IGNORECASE | Regexp::EXTENDED )
=> Regexp(/world/mix)
Finding matches
String.=~(Regexp) returns the starting position of the first match or nil if no match was found:
>> "123 456 789" =~ /\d+/
=> 0
>> "abc def ghi" =~ /\d+/
=> nil
>> if "123 456 789" =~ /\d+/ then "found" end
=> "found"
Special $ variables will contain information about the last match:
>> "123 456 789" =~ /(\d)(\d)(\d)/
=> 0
# $` contains text before last match
# $& contains last matched string
# $' contains text after last match
>> "#{$`}[#{$&}]#{$'}"
=> "[123] 456 789"
# $n contains the n-th (...) capture of the last match
>> "#{$1}-#{$2}-#{$3}"
=> "1-2-3"
# $~ contains MatchData object
>> $~.captures
=> ["1", "2", "3"]
String.scan returns all matches as String array:
# regex without capture group
>> "123 456 789".scan(/\d+/)
=> ["123", "456", "789"]
# assigning matches to variables
>> first, second, third = "123 456 789".scan(/\d+/)
# regex with captures
>> "123 456 789".scan(/(\d)(\d)(\d)/)
=> [["1", "2", "3"], ["4", "5", "6"], ["7", "8", "9"]]
String.scan can also be used with a block:
# regex without captures
>> "123 456 789".scan(/\d+/) {|m| p m }
"123"
"456"
"789"
# regex with captures
>> "123 456 789".scan(/(\d)(\d)(\d)/) { |m| puts "#{m.inspect}, 1st capture: #{$1}" }
["1", "2", "3"], 1st capture: 1
["4", "5", "6"], 1st capture: 4
["7", "8", "9"], 1st capture: 7
Replacing matches
String.gsub returns a new String with matches replaced, String.gsub! changes the String directly:
# replacing with a string (use \1, \2 ... to refer to captures)
>> "123 456 789".gsub(/(\d+)/, '[\1]')
>> "123 456 789".gsub(/(\d+)/, "[\\1]")
=> "[123] [456] [789]"
# replacing with a block (using $ variables)
>> "123 456 789".gsub(/(\d+)/) { |m| m.to_i * 2 }
>> "123 456 789".gsub(/(\d+)/) { $1.to_i * 2 }
=> "246 912 1578"
