Regex matching plus or minus

Question

Could someone please look at the following function and explain the regex for me as I don't understand it and I don't like using something I don't understand as then I won't be able to replicate it for use in the future and nor do I learn from it.

Also can someone explain the double !! in front, I know single means not so does double mean not "not"?

The function is a patch to String to check if it's capable of being converted to an integer or not.

class String 
    def is_i?
        !!(self =~ /\A[-+]?[0-9]+\z/) 
    end
end

The main thing that's giving me trouble is [-+] as it makes little sense to me, if you could explain in the context given it would be very helpful.

EDIT: Since people missed the second part of the question I'll be a little more explicit.

What does !! Mean in front of the check, I know a single ! means NOT but I can't find what !! means.

Answer 1

The [-+] Character Class

[-+] is a character class. It means "match one character specified by the class", i.e. - or +.

Hyphens in Character Classes

I can see how this particular class can be confusing because the hyphen often plays a special role in a character class: it links two characters to form a character range. For instance, [a-z] means "match one character between a and z, and [a-z0-9] means "match one character between a and z or between 0 and 9.

However, in this case, the hypen in [-+] is positioned in a place where it cannot be used to specify a range, and the - is just a literal hyphen.

Decoding the entire expression

• Assert position at the beginning of the string \A
• Match a single character from the list “-+” [-+]?
  • Between zero and one times, as many times as possible, giving back as needed (greedy) ?
• Match a single character in the range between “0” and “9” [0-9]+
  • Between one and unlimited times, as many times as possible, giving back as needed (greedy) +
• Assert position at the very end of the string \z

Answer 2

A Character Class defines a set of characters, any one of which can occur in a string for a match to succeed.

For example, the regular expression [-+]?[0-9]+ will match 123, -123, or +123 because it defines a character class (accepting either -, +, or neither one) as its first character.

In context:

• \A asserts position at the start of the string.
• [-+] any character of: - or + (? optional, meaning between zero and one time)
• [0-9] any character of: 0 to 9 (+ quantifier meaning 1 or more times)
• \z asserts position at the very end of the string.

What does !! mean?

!! placed together converts the value to a boolean.

Answer 3

explain the regex for me as I don't understand it

Pattern explanation: \A[-+]?[0-9]+\z

\A       Start of string
[-+]?    plus or minus sign [zero or one time (optional)]
[0-9]+   0 to 9 any digit [one or more times]
\z       End of string

The above regex pattern is able to match any positive and negative integer number that has + or - sign optional.

Read more about Character Classes and test your regex pattern online at Rubular