m/AP(?:\W|$)/i;

Approximately translated: match case insensitively AP, followed by either a non-word character, or the end of the string.

This makes the assumption that AP should match unless it's immediately followed by an alpha character.

You could also just force a word boundary and have a similar outcome:

m/AP\b/i

Updated: Added a colon in the appropriate place.

Second update: It's been mentioned to me that it is possible that the OP wants a word boundary before AP. Though it wasn't explicit in his writeup, you could use either of the two RE's to make that happen:

m/\bAP(?:\W|$)/i;
#or easier...
m/\bAP\b/i;
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