How to Find Index of Substring in Ruby: Simple Guide

In Ruby, you can find the index of a substring using the String#index method. It returns the position of the first occurrence of the substring or nil if the substring is not found.

Syntax

The String#index method is used to find the position of a substring inside a string.

• string.index(substring): Returns the index of the first occurrence of substring.
• If the substring is not found, it returns nil.

ruby

string.index(substring)

Example

This example shows how to find the index of a substring inside a string. It prints the index if found, or a message if not found.

ruby

text = "hello world"
index = text.index("world")
if index
  puts "Substring found at index: #{index}"
else
  puts "Substring not found"
end

Output

Substring found at index: 6

Common Pitfalls

One common mistake is expecting String#index to return -1 when the substring is not found. Instead, it returns nil. Always check for nil before using the result.

Also, String#index returns the first occurrence only. To find all occurrences, you need a different approach.

ruby

text = "hello world"
index = text.index("ruby")
if index.nil?
  puts "Substring not found"
else
  puts "Substring found at index: #{index}"
end

Output

Substring not found

Quick Reference

Method	Description	Return Value
string.index(substring)	Finds first index of substring	Integer index or nil if not found
string.index(substring, start)	Finds index starting from position start	Integer index or nil
string.rindex(substring)	Finds last index of substring	Integer index or nil

Key Takeaways

Use String#index to find the first position of a substring in Ruby.

If the substring is not found, String#index returns nil, not -1.

Always check for nil before using the index result to avoid errors.

To find the last occurrence, use String#rindex.

For multiple occurrences, you need to loop or use regular expressions.