GREP styles vs style sheets

Question

I've got a lull at work and decided to work on my list and start to learn GREP so that I can use GREP styles.
The first one is a simple one:  [[A-Z][A-Z]\S+?] This finds anything inside the brackets that starts with two capital letters. I know it works because I can find "[MT2]" in Find/Change.
OK, so I try it in some ID cs5 text with my first GREP style. It's failing the vast majority of the time. So I try lots of different things to see if I can find a pattern of what works and what doesn't. I've tried just the local formatting of a lone 'graf. I've tried making character and 'graf style sheets that incorporate a GREP style with the new character style, say "change type color to green".
I've looked through the forums and don't see others with my problem so it must be something about me or my document. I've used other documents to eliminate that variable. I've gone through the tutorials and David Blatner's stuff and don't see anything I'm doing differently.
When I format some text with a character and 'graf style sheet, it fails. BUT when I change the character style sheet of the source text to "[None]" it works. I've tried starting with text that is '[None]' and a 'graf style sheet of Normal, it works.
But starting with text styled with '[None]' character and a formal 'graf style sheet with a GREP Style, it fails. But when I change the 'graf style sheet to ]'Basic Paragraph]' it works.
I'm trying to find a pattern here, so to speak, and the only thing that has a chance to succeed is setting the character style sheet to 'None.' I just don't see than anyone who uses GREP styles has any problems like I do.
What do you reckon I'm overlooking? I've spent today trying to find what works and doesn't. As it is, I can't recommend this to my co-workers.

Answer 1

Rule of thumb for applying styles is basic formats for the entire paragraph should be defined in the paragraph style. If some selected bit of text needs to be formatted differnently, then you apply a character style (or, if you must, a local formatting override). With Nested Styles and GREP styles, ID has very powerful tools for applying character styles to selected text WITHIN the confines of the paragraph style definition. Most programs don't have that capability.
Should you apply the character syle as a nested or GREP style, or perhaps through Find/Change, or just by manually selecting text and applying the style? The answer to that will depend on a few variables. Should the particular string ALWAYS have a particular set of attributes, no matter what? Find/Change or local selection is the way to go. Character styles applied that way will survive even if the underlying paragraph style is changed to match the character style, then changed again. If the style should be applied to a string in a particular position in a paragraph, even if that position might move around a bit, a nested style is probably the way to go. If you want to make changes to text that matches a particular pattern, wherever it falls, when the surrounding text is a particular style, a GREP style is the answer.
GREP styles are very powerful, but they also use far more in terms of resources than applying a character style using other methods. I've seen reoprts of ID getting bogged down by too many GREP styles.