As your question currently stands there are many regular expressions that would match this but may be of no use. You will need to clarify your question and it's context to get a good answer. A couple of thoughts:

So you need to match three separate strings?
• A12345
• 12
• FB123

Will they appear consecutively? IE: A12345 12 FB123
OR
Something like this: Employee ID: A12345, Number of sick days: 12, Date of hire: 1.23.2011, Desk Location: FB123

Those two scenarios will provide very different answers. So to improve how you present your question a little bit here are a few things you might consider:
• Describe the data containing your strings.
  • Are they in an Excel, text or HTML document?
  • Are there multiple files you will be searching?
• Be as specific as possible about the strings you're trying to match:
  • Are the amount of characters in each string fixed or will they vary?
  • Will the strings appear consecutively, in the same order or appear more randomly?
  • Is there data that will consistently precede or follow these strings?

Also please include your attempts so far at writing a regular expression for the problem you're having.

Hope this helps. Good luck.