I tried %_filename% MATCHES ^\l but that seems to return any filename that starts with a word character; it is not bringing me only filenames that start with lowercase letters. Grrr, what to do?
Use:
%_filename% MATCHES (?-i)^\l
You have to force the search function within mp3tag to search case sensitive. Not sure if its a bug or not but the (?-i) matches the remainder of the regexp as case sensitive.
If and I assume you want to change the 1st letter of the string to UPPER case then here is the action to match:
Action type: Replace with regular expression
Field: _FILENAME
Regular expression: ^([a-z])
Replace matches with: $upper($1)
Make sure case sensitive comparison is checked.
Thanks =) its actually because I was trying to delete spaces, hypens, periods and numbers that were before any of those characters but I ended up deleting that plus the first character of every single filename! I've just been semi-manually changing them all back (mp3tag crashed when I tried to undo changes). Sigh. I haven't had the time to learn codes so thanks very much for the reply.
Opps! 
But as the saying goes you always learn the hard way. So a good rule of thumb is to test on dummy/single files first.
Dear @Florian, i think, this should be included in the help.
By the way, [1] is not a solution for non latin characters. But \l find them.
And: you must to repeat in every filter part.
This one does not work:
probleme IS "Lowercase name" OR %composer% MATCHES (?-i)^\l OR %arranger% MATCHES ^\l OR %lyricist% MATCHES ^\l OR %albumartist% MATCHES ^\l OR %artist% MATCHES ^\l OR %conductor% MATCHES ^\l
This one does:
probleme IS "Lowercase name" OR %composer% MATCHES (?-i)^\l OR %arranger% MATCHES (?-i)^\l OR %lyricist% MATCHES (?-i)^\l OR %albumartist% MATCHES (?-i)^\l OR %artist% MATCHES (?-i)^\l OR %conductor% MATCHES (?-i)^\l
Unfortunately, this second variant is not perfect, too: it does not find lowercase in the second, third composer/artist etc. It seems, line start (^) means the whole list's start, in this case. I could not find a solution. I tried even this:
"$ifgreater($strstr($regexp($meta_sep(composer,â),'â(?-i)\l',â,0),â),0,1,)" IS 1
...with no success. (It finds all files with multiple composer, even with uppercase start.)
To repeat the filter for all, from $meta(composer,0) to $meta(composer,10) is not very comfortable. All right, i do now a temporarly filter expression, repeating all for some possible items (composer 1st, 2nd, 3rd...), but it would be better, if somebody could tell a nice, real solution.
So, anyone, who needs it, can copy this temporary filter:
probleme IS "Lowercase name" OR "$meta(composer,0)" MATCHES (?-i)^\l OR "$meta(composer,1)" MATCHES (?-i)^\l OR "$meta(composer,2)" MATCHES (?-i)^\l OR "$meta(composer,3)" MATCHES (?-i)^\l OR "$meta(composer,4)" MATCHES (?-i)^\l OR %arranger% MATCHES (?-i)^\l OR "$meta(lyricist,0)" MATCHES (?-i)^\l OR "$meta(lyricist,1)" MATCHES (?-i)^\l OR %albumartist% MATCHES (?-i)^\l OR "$meta(artist,0)" MATCHES (?-i)^\l OR "$meta(artist,1)" MATCHES (?-i)^\l OR "$meta(artist,2)" MATCHES (?-i)^\l OR "$meta(artist,3)" MATCHES (?-i)^\l OR "$meta(artist,4)" MATCHES (?-i)^\l OR "$meta(conductor,0)" MATCHES (?-i)^\l OR "$meta(conductor,1)" MATCHES (?-i)^\l
But, it is a good idea to get a small pillow, just before filtering a few tens of thousands of files.
a-z âŠī¸