If you need this sort of thing, it may be that you *do* need regular expressions. Surely you can design other parts of your spreadsheet so that they will work correctly even with regular expressions allowed in such contexts? Remember that you can use the backslash to escape characters that would otherwise be interpreted as regular expressions.
Alternatively, you could create an extra column, say D. (I'm assuming that your rightmost numbers are in column C, not part of the contents of column B, in fact.) Enter =LEFT(B1) into D1 and fill down; =SUMIF(D1:D4;"a";A1:A4) or =SUMIF(D1:D4;D5;A1:A4) or =SUMIF(D1:D4;LEFT(B5);A1:A4) will then achieve what you need. If preferred, column D can be hidden, excluded from a print range, or placed out of the way or even on another sheet.
I trust this helps.
Brian Barker