Ruby Substring Examples

Get substrings with ranges and indexes. Regular expressions can be used to get substrings.
Substring. A substring is a range of characters within an existing string. There is no substring method in Ruby. Instead we rely upon ranges and expressions.
Ranges and indexes. With a range, we use periods in between two numbers—the first and last index of the substring. With indexes, we use a comma between 2 numbers—a start and a count.
Range example. We begin with ranges. We first examine the string "apple," which has 5 chars numbered 0 through 4. With the first range, we get a substring of the first 4 chars.

Negative: For a negative end on a range, we count backwards from the length of the string.

Dots: The period character is used within ranges. We can use 2 or 3 periods. If you enjoy typing periods, 3 is better.

Tip: The colon is not supported within ranges—we must use period-ranges. Otherwise the syntax resembles Python.

Ruby program that uses substring # Index 0 = a # Index 1 = p # Index 2 = p # Index 3 = l # Index 4 = e value = "apple" # Get substring at indexes 0 through 3. # ... This is the first four characters. first_part = value[0..3] # Get substring at indexes 3 through 4. # ... This is the character at index 3. second_part = value[2..3] # Get substring past index three through end of string. last_part = value[3..-1] puts first_part puts second_part puts last_part Output appl pl le
Comma indexes example. This is another syntax for substrings. We specify 2 numbers separated by a comma. The first number is the start index. The second is the length.

Tip: With the comma substring syntax, we do not specify a range with two indexes. The second number is now a length (a character count).

Ruby program that uses substring comma syntax value = "love" # Get substring at index 1 with length 3. # ... First argument is start index, second is length. last_three = value[1,3] puts last_three # Get substring at index 1 with length 2. middle_two = value[1,2] puts middle_two Output ove ov
Regular expressions. We can get substrings with a regular expression argument in place of ranges or indexes. The substring returned is the match of the regexp.

Here: We specify that we want substrings that are three characters long and begin with the letter "a."

Result: The first substring expression returns "ace" which is found in "peace." The second returns "art."

Ruby program that uses regexp substrings data = "peace" # Get three-letter substring starting with lowercase "a." three_letters = data[/a../] puts three_letters data = "part" # Get three letter substring for a different string. three_letters = data[/a../] puts three_letters Output ace art Regexp pattern a The lowercase letter "a". . Any character.
Substring tests. We can use an expression for checking if a substring is contained within a string. We specify the exact substring in square brackets.

True: In the first example, the philosopher string contains the substring "lato" so the substring test returns true.

False: When the substring is not found within the string, false is returned. In this way we search strings.

Ruby program that uses substring test expression philosopher = "plato" # This substring is found, so the expression returns true. if philosopher["lato"] puts true end # This substring is not contained with the string. if philosopher["soc"] puts false end Output true
Assign. A part of a string can be changed by assigning a substring. We can assign to a range of a string, or specify a start and a length (separated by a comma).

Tip: Regexp and exact strings can also be used. The matching part of the string is replaced with the specified value.

Ruby program that changes substrings value = "coffee" # Change range from 2 to last index. # ... This replaces the substring. value[2..-1] = "ugh" puts value value = "abcd" # Change substring starting at index 0 with length 2. # ... Assign a new substring. value[0,2] = "xy_" puts value Output cough xy_cd
Assign, first match. In assigning a substring, only the first match is replaced. Another string method, like replace(), can be used to replace all matching instances.
Ruby program that assigns substrings value = "one one one" # Replace first instance of this substring. value["one"] = "two" puts value # A regexp also replaces the first (leftmost) instance found. value[/o\w\w/] = "two" puts value Output two one one two two one Regexp pattern o The lowercase letter "o". \w A word character (letter or digit).
Assign all instances. We must use the gsub method to replace all instances of a substring with another. The sub() method acts in the same way as assigning a substring.Sub, gsub
A review. A single character can be returned with a single number. A longer substring can be specified with a simple range or 2 indexes (a start and a count).
With more advanced expressions, though, like regexp, we can encode searching within a substring expression. This is a powerful yet concise way of extracting string data.
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