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]
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]
# Get substring at index 1 with length 2.
middle_two = value[1,2]
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../]
data = "part"
# Get three letter substring for a different string.
three_letters = data[/a../]
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.
# This substring is not contained with the string.
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"
value = "abcd"
# Change substring starting at index 0 with length 2.
# ... Assign a new substring.
value[0,2] = "xy_"
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"
# A regexp also replaces the first (leftmost) instance found.
value[/o\w\w/] = "two"
Outputtwo one one
o The lowercase letter "o".
\w A word character (letter or digit).
Between, before, after. Suppose we wish to locate substrings that are between, before or after other substrings. We can develop methods with index() to perform this task.Between, before, after
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.Strings