How to Use sub with Regex in Ruby: Simple Guide

In Ruby, you can use sub with a regular expression to replace the first occurrence of a pattern in a string. The syntax is string.sub(/pattern/, 'replacement'), where the regex matches the part to change and the replacement is the new text.

Syntax

The sub method replaces the first match of a regular expression in a string with a specified replacement.

• string: The original text you want to change.
• /pattern/: The regular expression that finds the text to replace.
• 'replacement': The new text that will replace the matched part.

ruby

string.sub(/pattern/, 'replacement')

Example

This example shows how to replace the first word starting with 'cat' in a sentence using sub with a regex.

ruby

sentence = "The cat chased the caterpillar."
new_sentence = sentence.sub(/cat\w*/, 'dog')
puts new_sentence

Output

The dog chased the caterpillar.

Common Pitfalls

One common mistake is expecting sub to replace all matches, but it only replaces the first one. Use gsub to replace all occurrences.

Also, forgetting to use regex delimiters / / can cause errors or unexpected behavior.

ruby

text = "apple apple apple"

# Wrong: Using sub but expecting all replaced
puts text.sub(/apple/, "orange")

# Right: Using gsub to replace all
puts text.gsub(/apple/, "orange")

Output

orange apple apple
orange orange orange

Quick Reference

• sub(/regex/, 'text'): Replace first match only.
• gsub(/regex/, 'text'): Replace all matches.
• Use /pattern/ to define regex.
• Replacement can be a string or a block for dynamic changes.

Key Takeaways

Use sub with regex to replace only the first matching part of a string.

Always wrap your pattern in / / to use regex with sub.

For replacing all matches, use gsub instead of sub.

You can use a block with sub for more complex replacements.

Remember sub returns a new string; it does not change the original string unless you use sub!.