It's a fairly simple regex, really:
$replacedString = preg_replace('/\b([A-Z]{3,})\b/', '$1: ', $string);
It works like this:
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\b: word boundary. This detects the start and end of a "word"
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([A-Z]{3,}): Match 3 or more upper-case characters. The brackets capture this part of the match, so we can use it in the replacement string
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\b: Another word boundary
Replace this match with:
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'$1: ': the $1 refers back to the first captured group (the 3 or more upper case characters). To this, we're adding a colon and a space. That will be our replacement string
This will add the colon and space after all upper-case words of 3 or more characters. To replace only 1 word, just pass a limit to preg_replace:
$replaced = preg_replace('/\b([A-Z]{3,})\b/', '$1: ', $string, 1);
Where that last argument is the number of matches you wish to replace. -1 for all, 1 for 1, 2 for 2, etc...
Judging by your sample string, the upper-case words are city names. It's possible for city names to contain a dash, or even a space. To address this, you might want to match all strings containing upper-case chars, dashes and spaces:
$replaceAll = preg_replace('/\b([A-Z -]{2,}[A-Z])\b/', '$1: ', $string);
What changed:
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([A-Z -]{2,}: The capturing match start with upper-case chars (2 or more, not 3), but also matches spaces and dashes.
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[A-Z]): The last character of the captured group must be an upper-case character, this avoids capturing the trailing spaces or dashes. The result is that we capture stuff like "NEW YORK" or "FOO-TOWN", but not "ON - Something".
The rest is the same as before. If you want to allow for other characters that might occur (like a dot) just add them to the first part of the capturing group. The most complete pattern will probably be something like this:
$replaced = preg_replace('/\b([A-Z][A-Z .-]+[A-Z])\b/', '$1: ', $string);
This ensures the captured group starts, and ends with an upper case character, and contains any number of upper-case chars, spaces, dots and dashes in between. So this will match something like "ST. LEWIS", too
