How to write BBS format conversion code?

Hello,

I would like to write a simple forum for my site, which
enables some formating, like [b], [i], [u] and some other
BBS format code.

For example,
the user inputs this string in a multiline textbox, as
the body of his/her post,

[a=www.microsoft.com]Hello world[a]

The application will translate it to HTML code when his/
her post is shown up in the forum.

<a href="www.microsoft.com"><b>Hello <i>world</i></b></a>

Maybe I should use Regular Express to perform several
Replace methods on the string. But I am quite afraid of
the performance, as I were to support more and more
formatting codes besides the [a], [b], [i]...

Anybody have some opinions about how to write such kind of
format translation codes efficiently? Or where else I can
find some existing codes to begin with?

Thanks,
W. Jordan