Regular Expressions Cookbook, 2nd Edition

4.17. Find Addresses with Post Office Boxes

Problem

You want to catch addresses that contain a P.O. box, and warn users that their shipping information must contain a street address.

Solution

Regular expression

^(?:Post(?:al)?●(?:Office●)?|P[.●]?O\.?●)?Box\b

Regex options: Case insensitive, ^ and $ match at line breaks

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

C# example

Regex regexObj = new Regex(
    @"^(?:Post(?:al)? (?:Office )?|P[. ]?O\.? )?Box\b",
    RegexOptions.IgnoreCase | RegexOptions.Multiline
);
if (regexObj.IsMatch(subjectString) {
    Console.WriteLine("The value does not appear to be a street address");
} else {
    Console.WriteLine("Good to go");
}

See Recipe 3.5 for help with running a regular expression match test like this with other programming languages. Recipe 3.4 explains how to set the regex options used here.

Discussion

The following explanation is written in free-spacing mode, so each of the meaningful space characters in the regex has been escaped with a backslash:

^                # Assert position at the beginning of a line.
(?:              # Group but don't capture:
  Post(?:al)?\   #   Match "Post " or "Postal ".
  (?:Office\ )?  #   Optionally match "Office ".
 |               #  Or:
  P[.\ ]?        #   Match "P" and an optional period or space character.
  O\.?\          #   Match "O", an optional period, and a space character.
)?               # Make the group optional.
Box              # Match "Box".
\b               # Assert position at a word boundary.

Regex options: Case insensitive, ^ and $ match at line breaks, free-spacing

Regex flavors: .NET, Java, XRegExp, PCRE, ...