regexp literals

slashes define a regular expression literally (inline)

foo_matcher = /foo/

The above regexp will match a string containing "foo" anywhere inside it

ooo_matcher = /oo*/

The above regexp will match a string containing "o" or "oo" or "ooo" (and so on) anywhere inside it

a more complicated regexp

    word_exp = /\b[a-z]*\b/i

• slashes delineate the pattern
• backslash b means "match a word boundary"
• brackets delineate a match of a single character (aka "character class")
• a hyphen z means "all the letters between a and z, inclusive"
• star means match any number (0 or more) of the previous character (or, as in this case, character class)
• slash i at the end means "ignore capitalization" (or "case insensitive")

So the above expression will match any word containing only normal English letters, in any combination of upper- or lowercase.

regexp operators: equal tilde

equal tilde returns the position in the string that matches, or nil if no match

>> "abcde" =~ /bcd/
=> 1

Note that the return value is truthy if the string is a match, and falsey if it's not, which lets you use it inside conditionals:

if ("abcde" =~ /bcd/)
  puts "yay! it matches!"
end

regexp globals

• After a successful match, some global variables are set
  • $~ is what it matched
  • $1 is the first substring match
  • $2 is the second substring match
  • etc.
  • substrings are defined with parentheses in the regexp

if "the quick brown fox" =~ /(quick).*(f..)/ then
  puts "The matching string was #{$~}"
  puts "The first substring was #{$1}"
  puts "The second substring was #{$2}"
end

Prints this:

The matching string was quick brown fox
The first substring was quick
The second substring was fox

See Also

• $1 and \1 in Ruby
• Class Regexp rubydoc section on "Capturing"

regexp operators: bang tilde

bang tilde returns false if the string matches, or true if it doesn't

>> "abcde" !~ /xyz/
=> true
>> "abcde" !~ /bcd/
=> false

some methods that can use regexes

String.split

String.[]

String.sub and String.gsub

>> "foobar".sub("foo", "baz")
=> "bazbar"
>> "foobar".sub(/foo/, "baz")
=> "bazbar"
>> "foobar".sub(/fo*/, "baz")
=> "bazbar"
>> "fooooooobar".sub(/fo*/, "baz")
=> "bazbar"
>> "fooooooobarfoo".sub(/fo*/, "baz")
=> "bazbarfoo"
>> "fooooooobarfoo".gsub(/fo*/, "baz")
=> "bazbarbaz"

Array.grep

>> "foo bar baz".split.grep(/f../)
=> ["foo"]
>> "foo bar baz".split.grep(/b../)
=> ["bar", "baz"]

MatchData

• If you need more control you can use the match method
• It returns a MatchData object which is rather complex
• See rubydoc for Regexp and MatchData

learning more

• http://rubular.com is a great interactive Ruby regexp toy
• the Pickaxe book has a just-the-facts-maam overview of regexps
• cheat regexp and cheat regex have slightly different cheatsheets
  • gem install cheat for command-line tool or see http://cheat.errtheblog.com
• http://www.regular-expressions.info/tutorialcnt.html has a good tutorial
• Very quick but nice video on how regular expressions are like FSAs and how to debug them visually.

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