• Case insensitive (of course)
• All words can all be separated by nothing, spaces, periods, slashes, dashes, or underscores. (So "PO Box 123", "P.O. Box 123", "PO-Box 123", "PO_Box123", etc. all match.)
• Some valid abbreviations are P/Post/Postal, O/Off/Office, and B/Box/Bx/Bin (as well as N/No/Num/Number with "#" optionally). The only exception is when using only "box" or "bin", you must use the full word without abbreviation (so "box" or "bin", not "b" or "bx").
• Valid combinations are Post Office Box, Post Box, Post Office, or just Box (all must have numbers). So it doesn't match "Post 123", "Postal 123", "Office 123", or "Mailbox 123". Currently we don't match for "mailbox", and there has to be more than just "post", "postal", or "office" in the address.
• Anything with a Box + number will match, with the exception of rural route addresses (like "RR 1 Box 2", "R.R. 23 Box 45", or "Rural Route Box 123"). So it also matches military APO/FPO address, like "PSC 123 Box 1234" (because of the Box + number).
• The PO Box must have a number after it in order to match. The number regex is very fuzzy, so it will match a lot of things, like "1", "#1", "No. # 1", "number 1", "Num 1", etc.

Code to generate the regex:

// Get the string version of each regex that we need to match PO boxes
const separator = /[ ./\-_]/.source; // Any of these characters can separate the PO box words (space, period, slash, dash, underscore)
const postalGroup = /(P(?:ost(?:al)?)?)/i.source; // "P", "Post", "Postal"
const officeGroup = /(O(?:ff(?:ice)?)?)/i.source; // "O", "Off", "Office"
const boxGroup = /(b(?:o?x|in)?)/i.source; // "b", "bx", "box", "bin"
const fullBoxGroup = /(box|bin)/i.source; // "box" or "bin" (full word only)
const ruralRouteGroup = /(r(?:ural)? r(?:oute)?)/i.source; // "rr", "r r", "ruralr", "rroute", "rural route", etc. (space will be replaced with full separators later)
const numTextGroup = /(n\.?(?:o|um(?:ber)?)?\.?)/i.source; // "n", "no", "num", "number", "n.", "n.o.", "no.", "num.", etc.
const digitsGroup = /(\d+)/.source; // 1 or more digits

// Spaces in below strings will be replaced later with the separator regex

// Construct number part of PO box regex string
const poBoxNumberGroup = `(${numTextGroup}? #* ${digitsGroup})`; // Match "number # 123", "no 123", "# 123", "123", etc.
// Construct PO box words regex string
const poBoxWordsOfficeOptional = `(?:${postalGroup} ${officeGroup}? ${boxGroup})`; // Match stuff like "post box", "post office box", or "P B" (where the office part is optional)
const poBoxWordsBoxOptional = `(?:${postalGroup} ${officeGroup} ${boxGroup}?)`; // Match stuff like "post office", "post office box", or "PO" (where the box part is optional)const ruralRouteNegativeGroup = /(?!r(?:ural)? r(?:oute)? (\d+)?)/i.source; // Negative lookahead for "rr 12", "r r 1", "ruralr 1", "rroute 1", "rural route 1", etc. (space will be replaced with full separators later)
const poBoxWordsBoxOnly = `(?:(?<!${ruralRouteGroup} ${poBoxNumberGroup}? )${fullBoxGroup})`; // Match just "box" or "bin" (unless preceded by a rural route)
const poBoxWordsGroup = `(${poBoxWordsOfficeOptional}|${poBoxWordsBoxOptional}|${poBoxWordsBoxOnly})`; // Match either of the above
// Construct the whole PO box regex string (with word boundaries, but still excluding the separators)
const wholePOBoxGroup = `\\b(${poBoxWordsGroup} ${poBoxNumberGroup})\\b`;

// Construct the final PO box regex
const PO_BOX_REGEX = new RegExp(
  wholePOBoxGroup.replaceAll(" ", `${separator}*`), // Replace all spaces with regex matching any number of the separators
  "i"
);

// Check if the address is a PO box
export function isPOBox(addressString): boolean {
  return PO_BOX_REGEX.test(addressString);
}

// Extract the PO box part from the address
export function getPOBox(addressString) {
  const match = addressString?.match(PO_BOX_REGEX);
  return match ? match[1] : null;
}