How to Replace Text in a String in Ruby: Simple Guide

In Ruby, you can replace text in a string using the String#sub method to replace the first occurrence or String#gsub to replace all occurrences. Both methods return a new string with replacements, while their bang versions sub! and gsub! modify the original string in place.

Syntax

The main methods to replace text in Ruby strings are:

• string.sub(pattern, replacement): replaces the first match of pattern with replacement.
• string.gsub(pattern, replacement): replaces all matches of pattern with replacement.
• sub! and gsub! are the same but change the original string directly.

ruby

string.sub(pattern, replacement)
string.gsub(pattern, replacement)
string.sub!(pattern, replacement)
string.gsub!(pattern, replacement)

Example

This example shows how to replace the first and all occurrences of a word in a string using sub and gsub. It also shows how to modify the original string with gsub!.

ruby

text = "I like apples. Apples are tasty."

# Replace first occurrence (case sensitive)
new_text = text.sub("apples", "oranges")
puts new_text

# Replace all occurrences (case sensitive)
new_text_all = text.gsub("apples", "oranges")
puts new_text_all

# Replace all occurrences ignoring case
new_text_ignore_case = text.gsub(/apples/i, "oranges")
puts new_text_ignore_case

# Modify original string
text.gsub!("apples", "oranges")
puts text

Output

I like oranges. Apples are tasty.
I like oranges. Apples are tasty.
I like oranges. Oranges are tasty.
I like oranges. Oranges are tasty.

Common Pitfalls

Common mistakes when replacing strings in Ruby include:

• Using sub when you want to replace all occurrences instead of just the first.
• Not using regular expressions when you want case-insensitive replacements.
• Expecting sub or gsub to modify the original string without using the bang versions sub! or gsub!.

ruby

text = "Hello hello hello"

# Wrong: only replaces first occurrence
puts text.sub("hello", "hi")

# Right: replace all occurrences ignoring case
puts text.gsub(/hello/i, "hi")

# Wrong: expecting original string changed
text.sub("Hello", "Hi")
puts text

# Right: modify original string
text.sub!("Hello", "Hi")
puts text

Output

Hello hi hello
hi hi hi
Hello hello hello
Hi hello hello

Quick Reference

Method	Description	Modifies Original String?
sub(pattern, replacement)	Replace first match, returns new string	No
gsub(pattern, replacement)	Replace all matches, returns new string	No
sub!(pattern, replacement)	Replace first match, modifies original string	Yes
gsub!(pattern, replacement)	Replace all matches, modifies original string	Yes

Key Takeaways

Use sub to replace the first occurrence and gsub to replace all occurrences in a string.

Add ! to modify the original string instead of returning a new one.

Use regular expressions with i flag for case-insensitive replacements.

Remember sub and gsub return new strings and do not change the original unless you use sub! or gsub!.

Test your replacement to avoid unexpected results with case sensitivity or partial matches.