Regular expressions (regex) are a powerful way to search, match, extract, and replace text in Ruby. Ruby has first-class regex support built right into the language, making text processing concise and expressive.
This tutorial explains Ruby regular expressions from the basics to practical real-world patterns, with plenty of clear examples.
1) What Are Regular Expressions in Ruby?
A regular expression is a pattern used to match text.
In Ruby, regex patterns are usually written between slashes:
/pattern/
Example:
puts "hello".match(/ell/)
# => #<MatchData "ell">
2) Matching Text (match, =~)
Using match
text = "Ruby is fun"
if text.match(/Ruby/)
puts "Match found"
end
Using =~
puts "hello" =~ /ell/
# => 1 (starting index)
Returns nil if no match.
3) Character Classes
Basic character sets
/[abc]/ # a, b, or c
/[a-z]/ # lowercase letters
/[A-Z]/ # uppercase letters
/[0-9]/ # digits
Example:
puts "cat" =~ /[aeiou]/
# => 1
Negated character class
/[^0-9]/ # anything except digits
4) Quantifiers
| Quantifier | Meaning | | – | | | * | 0 or more | | + | 1 or more | | ? | 0 or 1 | | {n} | exactly n | | {n,} | n or more | | {n,m} | between n and m |
Example:
puts "aaa" =~ /a+/
# => 0
5) Anchors (^ and $)
Anchors match positions, not characters.
/^hello/ # start of string
/world$/ # end of string
Example:
puts "hello world" =~ /^hello/
# => 0
puts "hello world" =~ /world$/
# => 6
6) Capturing Groups and MatchData
Capturing groups
match = "2025-12-25".match(/(\d{4})-(\d{2})-(\d{2})/)
puts match[1] # year
puts match[2] # month
puts match[3] # day
Named captures
match = "2025-12-25".match(/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/)
puts match[:year]
puts match[:month]
puts match[:day]
7) Alternation (|)
Match one pattern or another.
puts "cat" =~ /cat|dog/
puts "dog" =~ /cat|dog/
8) Substitution (sub and gsub)
Replace first match
text = "I like apples and apples"
puts text.sub(/apples/, "oranges")
# => I like oranges and apples
Replace all matches
puts text.gsub(/apples/, "oranges")
# => I like oranges and oranges
9) Splitting Strings with Regex
data = "one,two;three four"
parts = data.split(/[,; ]/)
p parts
# => ["one", "two", "three", "four"]
10) Common Real-World Regex Examples
Validate an email (basic)
email = "user@example.com"
if email =~ /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i
puts "Valid email"
end
Remove extra whitespace
text = " Ruby is fun "
clean = text.gsub(/\s+/, " ").strip
puts clean
# => Ruby is fun
Extract numbers from a string
text = "Order #123 costs $45"
numbers = text.scan(/\d+/)
p numbers
# => ["123", "45"]
11) Regex Options (Flags)
| Flag | Meaning |
|---|---|
i |
case-insensitive |
m |
multiline |
x |
extended (whitespace allowed) |
Example:
puts "RUBY" =~ /ruby/i
12) Best Practices for Ruby Regex
- Keep patterns readable
- Use named captures for clarity
- Avoid overly complex expressions
- Test regex with real input
- Comment complex patterns
FAQ
Q: How do I use regex in Ruby? A: Use patterns like /text/ with methods such as match, =~, sub, and gsub.
Q: What is MatchData in Ruby? A: MatchData holds the result of a regex match, including captures and match positions.
Q: Are Ruby regexes the same as PCRE? A: Ruby regex is similar to PCRE but has Ruby-specific features like named captures and global variables.