^\s*                #Line start, match any whitespaces at the beginning if any.
(?:\+?(\d{1,3}))?   #GROUP 1: The country code. Optional.
[-. (]*             #Allow certain non numeric characters that may appear between the Country Code and the Area Code.
(\d{3})             #GROUP 2: The Area Code. Required.
[-. )]*             #Allow certain non numeric characters that may appear between the Area Code and the Exchange number.
(\d{3})             #GROUP 3: The Exchange number. Required.
[-. ]*              #Allow certain non numeric characters that may appear between the Exchange number and the Subscriber number.
(\d{4})             #Group 4: The Subscriber Number. Required.
(?: *x(\d+))?       #Group 5: The Extension number. Optional.
\s*$                #Match any ending whitespaces if any and the end of string.