Replace hyperlinks from md to bb
\[(.*?)\]\((.*?)\)
[URL="$2"]$1[/URL]

Replace bold from md to bb
\*\*(.*?)\*\*
[b]$1[/b]

Replace heading 3 from md to bb
### (.*?)$
[SIZE=5]$1[/SIZE]

Replace heading 2 from md to bb
## (.*?)$
[SIZE=6]$1[/SIZE]

Replace heading 1 from md to bb
# (.*?)$
[SIZE=7]$1[/SIZE]