Telephone Number Validator (Reg Ex)

Telephone Number Validator.txt

ASSIGNMENT
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Return true if the passed string looks like a valid US phone number.

The user may fill out the form field any way they choose as long as it has 
the format of a valid US number. The following are examples of valid formats for US numbers 
(refer to the tests below for other variants):

    555-555-5555
    (555)555-5555
    (555) 555-5555
    555 555 5555
    5555555555
    1 555 555 5555

For this challenge you will be presented with a string such as 800-692-7753 or 8oo-six427676;laskdjf. 
Your job is to validate or reject the US phone number based on any combination of the formats provided above. 
The area code is required. If the country code is provided, you must confirm that the country code is 1. 
Return true if the string is a valid US phone number; otherwise return false.
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SOLUTION
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function telephoneCheck(str) {
  // Good luck!
  
  var regex = /^(1\s?)?(\(\d{3}\)|\d{3})[\s\-]?\d{3}[\s\-]?\d{4}$/;
  return regex.test(str);  
}

telephoneCheck("1 555 555 5555");

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EXPLANATION
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^ matches the beginning of our string
(1\s?)? means optional "1", followed or not followed by a space (here, '?' stands for "optional")
(\(\d{3}\)|\d{3}) means three digits, inside paranthesis or (|) three digits without paranthesis
[\s\-]? means optional space or dash
$ matches end of input (so that there is nothing after the last four digits)