Global Search? I'm not impressed by it. No literal string support. You can't even tell it you want to search just message subject lines.

I use FiltaQuilla and its regular expressions capability. Unfortunately it only offers support for the in-folder search.

So to work round this, I'd set up a Saved Search, using

subjectre + matches + /a friend of/i

and configure it to search all folders. The /…/i configures it to ignore case.

Sorry, but what program language is this? Could you parse this expression?

Right, let's backstep a bit. Right now I don't think you need regular expressions or FiltaQuilla.

Judging by the disappointing search results, I'm guessing that you are using the Global Search; the prominent search box at top right. It's OK, but can't do precise searches.

If you know which folder you need to search in, you can use an older search feature. Select your folder and use ctrl+shift+f to search in it, and you'll see a dialogue that allows you to specify where to search, how to search and what to search for. So you could enter:

subject as where to search, contains as to how to search, and a friend of as the target to search for.

So we could summarize this as: subject + contains + a friend of

This will search for the exact string specified, ignoring case. So we have a more precise search, looking only into the subject line.

But if you don't know which folder to search in, we can use another trick to create a search across multiple folders. This is the Saved Search Folder feature.

Open Local Folders. Use File|New|Saved Search…. You'll see a dialogue similar to that used for searching a folder, but with a button that lets you choose which folders to search. "All" will obviously select all your mail folders. Give it a name, at the top, so you know what it does. Click "Create" and it will build and populate a new virtual folder containing all the messages that meet your search conditions.

You can set up multiple rules in the search dialogue, but you need to think about whether you want it to fire only when all conditions are met, ("match all of the following") or if any one of the conditions will do the job ("match any of the following"). For some reason, these two settings confuse many users.

You could keep a Saved Search folder to hand, and just edit (right-click|properties) it to change the search target or rules. Or create and destroy it as needed.

Now the main weakness of this older search is that it searches for precisely what you give it and sometimes you need a bit of woolliness. And another important issue is that Thunderbird's filters are blind to certain characters. I know of + and & as things it can't see (there may be more), and if you're searching for correspondence from a business named "A&B Solutions", you won't be able to search for the &. In this case a search for contains + B solutions would probably do the job. You need to be creative in your thinking. But when you need to go beyond this, then regular expressions are a powerful pattern matching tool that allow you to search for variations on a specific string, and here you'll need FiltaQuilla to provide regular expression support.

So, if your search was for either "a friend of" or "some friends of", then you could have two rules in your search, with the "match any" setting:

subject + contains + a friend of subject + contains + some friends of

or you could use a regular expression such as

subjectre + matches + /(a|some) friends? of/i

a|some: either a or some s?: optional s /…/i: ignore case

This is a little less precise than the two-rule solution, as it would also find "some friend of", but that's probably not enough of a nuisance to matter. It's a bit less typing and often an interesting challenge to phrase your search terms.

On top of all this, there is also the QuickFilter bar which lets you search within a folder. Again, it is not too precise, but can be enhanced by use the of the Expression Search / Google Mail UI addon, which, again, lets you set the scope of the search. And supports regular expressions. ;-)

Thanks! Very helpful!