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Ruby Substring Examples

These Ruby examples get substrings with ranges and indexes. They acquire substrings with regular expressions.

Substrings. 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 two numbers—a start and a count.

Range example. We begin with ranges. We first examine the string "apple," which has five chars numbered 0 through 4. With the first range, we get a substring of the first four 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.

Based on:

Ruby 2

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



Comma indexes example. This is another syntax for substrings. We specify two 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



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



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

# This substring is not contained with the string.
if philosopher["soc"]
    puts false



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



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


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 two 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.