Searching for Substrings

Problem

You need to search for a substring, and return either the starting position of the match or the matching value itself.

Solution

There are several ways to accomplish this using regular expressions. Some methods are called on a RegExp pattern or object and some are called on String objects.

RegExp objects

The first way is to call the test method on a RegExp pattern or object. The test method returns a boolean value:

match = /sample/.test("Sample text")
# => false

match = /sample/i.test("Sample text")
# => true

The next way to is to call the exec method on a RegExp pattern or object. The exec method returns an array with the match information or null:

match = /s(amp)le/i.exec "Sample text"
# => [ 'Sample', 'amp', index: 0, input: 'Sample text' ]

match = /s(amp)le/.exec "Sample text"
# => null

String objects

The match method matches a given string with the RegExp. With ‘g’ flag returns an array containing the matches, without ‘g’ flag returns just the first match or if no match is found returns null.

"Watch out for the rock!".match(/r?or?/g)
# => [ 'o', 'or', 'ro' ]

"Watch out for the rock!".match(/r?or?/)
# => [ 'o', index: 6, input: 'Watch out for the rock!' ]

"Watch out for the rock!".match(/ror/)
# => null

The search method matches RegExp with string and returns the index of the beginning of the match if found, -1 if not.

"Watch out for the rock!".search /for/
# => 10

"Watch out for the rock!".search /rof/
# => -1

Discussion

Regular Expressions are a powerful way to test and match substrings.