\+?optionally matches the literal + (4310 / 538 / 2B16)
\da digit
\h*any horizontal whitespace character, repeated any number of times
\(the literal ( (4010 / 508 / 2816)
\d{3}a digit, repeated exactly 3 times
\)the literal ) (4110 / 518 / 2916)
\h*any horizontal whitespace character, repeated any number of times
+matches at least one character from the set below:
\da digit
-the literal - (4510 / 558 / 2D16)
g modifier: global. Finds all matches instead of stopping after the first
m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string