Ruby | MatchData Class

In Ruby, all the pattern matching is done with the help of special variable $~. All the pattern matches will set the $~ to a MatchData that contains the information about the match. The MatchData objects are returned by the Regexp#match and Regexp.last_match methods. The MatchData objects enclosed all the result of the pattern match, results that are normally accessed by the special variables, i.e $&, $’, $`, $1, $2 etc. 

Instance Method

Here match is the object of the MatchData Class.

• [] : It is known as Match Reference. In this MatchData is behaves like an array and may be accessed by using normal array indexing technique. In this match[0] is equivalent to special variable $& and it returns the entire matched string. match[1], match[2], match[3] and so on, return the match back reference value.

match[i]
match[start, length]
match[range]

Example:

Ruby

# Ruby program to illustrate
# use of []

# Using [] operator
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks12.")
a[0]
a[1, 4]
a[1..2]
a[-2, 1]

Output:

"ks12"
["k", "s", "1", "2"]
["k", "s"]
["1"]

• begin : This method returns the offset of the start of the nth element of the match array in the string.

match.begin(n)

Example:

Ruby

# Ruby program to illustrate
# use of begin method

# Using begin method
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks112.")
a.begin(1)
a.begin(2)

Output:

11
12

• captures : This methods return the array of all the matching groups.

match.captures

Example:

Ruby

# Ruby program to illustrate
# use of captures method

# Using captures method
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks112.")
a.captures

Output:

["k", "s", "11", "2"]

• end : This method returns the offset into the character that immediately following the end of the nth element of thematch array in the string.

match.end(n)

Example:

Ruby

# Ruby program to illustrate
# use of end method

# Using end method
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks112.")
a.end(0)
a.end(2)

Output:

16
13

• length : This method returns the number of elements present in the match array.

match.length

Example:

Ruby

# Ruby program to illustrate
# use of length method

# Using length method
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks112.")
a.length

# using size method
a.size

Output:

5
5

• offset : This method return a two-element array that consist of starting and ending offset of the nth match.

match.offset(n)

Example:

Ruby

# Ruby program to illustrate
# use of offset method

# Using offset method
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks112.")
a.offset(2)
a.offset(1)

Output:

[12, 13]
[11, 12]

• post_match : This method returns the piece of the original string after the current match. Identical to the special variable $’.

match.post_match

Example:

Ruby

# Ruby program to illustrate
# use of post_match method

# Using post_match method
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks112: Ruby")
a.post_match

Output:

": Ruby"

• pre_match : This method returns the piece of the original string before the currentmatch. Identical to the special variable $`.

match.pre_match

Example:

Ruby

# Ruby program to illustrate
# use of pre_match method

# Using pre_match method
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks112: Ruby")
a.pre_match

Output:

"GeeksFORgee"

• select : This method return an array that consist of all the elements of match for which the block is true.

match.select{|val|block}

• size : This method is similar to MatchData#length method.

match.size

• string : This method return a frozen copy of the string passed in the match.

match.string

Example:

Ruby

# Ruby program to illustrate
# use of string method

# Using string method
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks112: Ruby")
a.string

Output:

"GeeksFORgeeks112: Ruby"

• to_a : This method returns the array of matches.

match.to_a

Example:

Ruby

# Ruby program to illustrate
# use of to_a method

# Using to_a method
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks112: Ruby")
a.to_a

Output:

["ks112", "k", "s", "11", "2"]

• to_s : This method returns the whole matched string.

match.to_s

Example:

Ruby

# Ruby program to illustrate
# use of to_s method

# Using to_s method
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks112: Ruby")
a.to_s

Output:

"ks112"

• values_at : In this method index is used to access the matching values and return an array of corresponding match.

match.valu_at([index]*)

Example:

Ruby

# Ruby program to illustrate
# use of values_at method

# Using values_at method
a = /(.)(.)(\d+)(\d)/.match("GeeksFORgeeks112: Ruby")
a.values_at(2, 0)

Output:

["s", "ks112"]

• == : It is known as Equality. It is used to check if both MatchData, i.e match1 and match2 are equal or not. If they are equal, then return true, otherwise return false. 

match1==match2

Reference: https://ruby-doc.org/core-2.2.0/MatchData.html