^start of string
Group 1([+]?\d{1,2}[-\s]?|) 1st Alternative[+]?\d{1,2}[-\s]? ?matches at most one character from the set below:
+the literal + (4310 / 538 / 2B16)
\d{1,2}a digit, repeated between 1 and 2 times
?matches at most one character from the set below:
-the literal - (4510 / 558 / 2D16)
\sany whitespace character
2nd Alternative — matches an empty string
\d{3}a digit, repeated exactly 3 times
?matches at most one character from the set below:
-the literal - (4510 / 558 / 2D16)
\sany whitespace character
\d{3}a digit, repeated exactly 3 times
?matches at most one character from the set below:
-the literal - (4510 / 558 / 2D16)
\sany whitespace character
\d{4}a digit, repeated exactly 4 times
$end of string