(?i)applies the following effective flags to the remainder of the pattern: gmi i modifier: insensitive. Matches without distinguishing uppercase and lowercase letters
^start of line
\s*any whitespace character, repeated any number of times
Group 1(.*((p|post)[-.\s]*(o|off|office)[-.\s]*(b|box|bin)[-.\s]*)|.*((p|post)[-.\s]*(o|off|office)[-.\s]*)|.*((p|post)[-.\s]*(b|box|bin)[-.\s]*)|(box|bin)[-.\s]*) 1st Alternative.*((p|post)[-.\s]*(o|off|office)[-.\s]*(b|box|bin)[-.\s]*) .*any character (except for line terminators), repeated any number of times
Group 2((p|post)[-.\s]*(o|off|office)[-.\s]*(b|box|bin)[-.\s]*) pthe literal p (11210 / 1608 / 7016) (case insensitive)
postthe literal text post (case insensitive) *matches any number of characters from the set below:
-the literal - (4510 / 558 / 2D16)
.the literal . (4610 / 568 / 2E16)
\sany whitespace character
othe literal o (11110 / 1578 / 6F16) (case insensitive)
offthe literal text off (case insensitive) officethe literal text office (case insensitive) *matches any number of characters from the set below:
-the literal - (4510 / 558 / 2D16)
.the literal . (4610 / 568 / 2E16)
\sany whitespace character
bthe literal b (9810 / 1428 / 6216) (case insensitive)
boxthe literal text box (case insensitive) binthe literal text bin (case insensitive) *matches any number of characters from the set below:
-the literal - (4510 / 558 / 2D16)
.the literal . (4610 / 568 / 2E16)
\sany whitespace character
2nd Alternative.*((p|post)[-.\s]*(o|off|office)[-.\s]*) .*any character (except for line terminators), repeated any number of times
Group 6((p|post)[-.\s]*(o|off|office)[-.\s]*) pthe literal p (11210 / 1608 / 7016) (case insensitive)
postthe literal text post (case insensitive) *matches any number of characters from the set below:
-the literal - (4510 / 558 / 2D16)
.the literal . (4610 / 568 / 2E16)
\sany whitespace character
3rd Alternative.*((p|post)[-.\s]*(b|box|bin)[-.\s]*) 4th Alternative(box|bin)[-.\s]* Group 13(#|n|num|number)? \s*any whitespace character, repeated any number of times
\d+a digit, repeated at least once
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