*Note to readers: LibreOffice added wildcards and regular expressions, generally compatible with other spreadsheet applications like Excel, around version 5. At the time of this question, I was using the latest version of LO in the Debian repository, which pre-dated that. If you are reading this now, the accepted answer is the best solution. I can think of only one case where you might want to use this answer as an alternative: if you're stuck with an ancient version of LO Calc (which sometimes happens with some featherweight distros that use custom packages). I'll leave this answer in place just in case.*

Here's a workaround that uses a helper column (it can be used without a helper column if your version of LO Calc supports SUMPRODUCT, which I'll show at the end).

You can hide column B. Cell B1 contains:

```
=ISNUMBER(SEARCH("a",A1))*ROW()
```

Copy that down the column as needed. This provides the row number if the cell contains "a", otherwise zero. The cell address that you want is in D1:

```
=ADDRESS(SUM(B1:B3),1)
```

The sum of the matching row and a bunch of zeros gives you the row, and the column is known. You could use these values directly inside a formula rather than creating a text representation of the address.

You can eliminate the helper column with SUMPRODUCT, in which case the formula in D1 would be:

```
=ADDRESS(SUMPRODUCT(ISNUMBER(SEARCH("a",A1:A3))*ROW(A1:A3)),1)
```

SUMPRODUCT treats the range as an array and does what the helper column does in one step.