What is wrong with this Regex in Ruby on Rails?

Question

I've written a regex to help validate a String for game character names. It's somehow passing seemingly invalid strings and not passing seemingly valid strings.

Requirements:

Starts with a capital letter
Has any number of alphanumeric characters after that (this includes spaces)

This is the rails code that does the validation in the Character Model:

validates :name, format: { with: %r{[A-Z][a-zA-Z0-9\s]*} }

Here's the unit test I'm using

test "character name should be properly formatted and does not contain any special characters" do
    character = get_valid_character
    assert character.valid?

    character.name = "aBcd"
    assert character.invalid?, "#{character.name} should be invalid"

    character.name = "Number 1"
    assert character.valid?, "#{character.name} should be valid"

    character.name = "McDonalds"
    assert character.valid?, "#{character.name} should be valid"

    character.name = "Abcd."
    assert character.invalid?, "#{character.name} should be invalid"

    character.name = "Abcd%"
    assert character.invalid?, "#{character.name} should be invalid"
end

The problems: The regex passes "aBcd", "Abcd.", and "Abcd%" when it shouldn't. Now, I know this works because I tested this out in Python and it works just as you would expect.

What gives?

Thank you for your help!

Jamison Dance · Accepted Answer · 2012-01-07 04:09:09Z

6

Regular expressions look for matches anywhere in the given string unless told otherwise.

So the test string 'aBcd' is invalid, but it contains a valid substring: 'Bcd'. Same with 'Abcd%', where the valid substring is 'Abcd'.

If you want to match the entire string, use this as your regex:

# \A matches string beginning, \z matches string end
%r{\A[A-Z][a-zA-Z0-9\s]*\z}

PS: Some people will say to match the beginning of a string with ^ and the end with $. In Ruby, those symbols match the beginning and end of a line, not a string. So "ABCD\n%" would still match if you used ^ and $, but won't match if you use \A and \z. See the Rails security guide for more on this.

answered Jan 7, 2012 at 4:09

Jamison Dance

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3 Comments

ruakh Over a year ago

Would that properly forbid newlines? Some Google-hits suggest that in Ruby, ^ and $ default to meaning "start-of-line" and "end-of-line" (like other languages' multiline-mode regexes), so you need to use \A and \z to mean "start-of-string" and "end-of-string". (I have not tested this myself.)

Jamison Dance Over a year ago

Rushing to get the answer, and I forgot about that gotcha. :)

Jamornh Over a year ago

Thank you that clears this up a lot! It works wonderfully now!

jstim · Accepted Answer · 2012-01-07 04:33:22Z

0

If you only want to match the capital letter at the beginning of the string, you need to put in the "start of line" marker ^ so it would look like:

validates :name, format: { with: %r{^[A-Z][a-zA-Z0-9\s]*} }

Check out Rubular to play around with your regex

edited Jan 7, 2012 at 4:33

answered Jan 7, 2012 at 4:10

jstim

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2 Comments

Jamison Dance Over a year ago

Rubular is actually where I tested the answer to this question. :) Great resource.

jstim Over a year ago

No problem. so close though, but Jergason's is exactly what you need. Good luck!

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