Ruby Regular Expressions

Regular ExpressionIt is a special sequence of characters that matches or searches for a set of strings using a pattern with a specific syntax.

Regular expressions are composed of some predefined special characters and combinations of these characters, forming a "rule string" that is used to express a filtering logic for strings.

Syntax

Regular ExpressionLiterally a pattern between slashes or between any delimiter following %r, as shown below:

　/pattern/
/pattern/im          # Options can be specified
%r!/usr/local! # Use a delimiter in the regular expression

Online Examples

#!/usr/bin/ruby
　
line1　= "Cats are smarter than dogs";
line2　= "Dogs also like meat";
　
if ( line1　=~　/Cats(.*)/　)
　　puts "Line1　contains Cats"
end
if ( line2　=~　/Cats(.*)/　)
　　puts "Line2　contains    Dogs"
end

The output of the above example is:

Line1　contains Cats

Regular Expression Modifiers

The regular expression may contain an optional modifier on the surface, used to control various aspects of matching. Modifiers are specified after the second slash character, as shown in the example above. The following list shows possible modifiers:

Just like strings are separated by %Q, Ruby allows you to use %r as the beginning of a regular expression, followed by any separator. This is very useful when describing a string with a large number of slashes that you do not want to escape.

# Below matches a single forward slash character, not escaped
%r|/|　　　　　　　　　　　　　　　
　
# Flag character can be matched by the following syntax
%r[</(.*)>]i

Regular expression pattern

Except control characters,(+ ? . * ^ $ ( ) [ ] { } | \)All other characters match themselves. You can escape control characters by placing a backslash before them.

The table below lists the regular expression syntax available in Ruby.

Regular Expression Examples

character

Character class

Special character class

Repetition

Non-greedy repetition

This will match the minimum number of repetitions.

Grouping by parentheses

Backreference

This will match the previously matched group again.

Replacement

Anchor

This requires specifying the matching position.

Special syntax of parentheses

Search and replace

sub and gsub and their replacement variables sub! and gsub! are important string methods when using regular expressions.

All these methods are used to perform search and replace operations using regular expression patterns.sub and sub! Replace the first occurrence of the pattern.gsub and gsub! Replace all occurrences of the pattern.

sub and gsub Returns a new string without modifying the original string. sub! and gsub! It will modify the strings they call.

Online Examples

#!/usr/bin/ruby
#　-*-　coding: UTF-8　-*-
　
phone = "138-3453-1111　# This is a phone number"
　
# Delete Ruby comments
phone = phone.sub!(/#.*$/, ")　　　
puts "Phone Number : #{phone}"
　
# Remove all characters except numbers
phone = phone.gsub!("/\D/, ")　　　　
puts "Phone Number : #{phone}"

The output of the above example is:

Phone Number :　138-3453-1111　
Phone Number :　13834531111

Online Examples

#!/usr/bin/ruby
#　-*-　coding: UTF-8　-*-
　
text = "rails is rails, Ruby on Rails is a very good Ruby framework"
　
# Change all "rails" to "Rails"
text.gsub!("rails", "Rails")
　
# Change all the words "Rails" to uppercase
text.gsub!("/\brails\b/, "Rails")
　
puts "#{text}"

The output of the above example is:

Rails is a very good Ruby framework for Ruby on Rails