Doubt with regex to replace square bracket

Shell Programming and Scripting
1

there is a word "welcome[world]"
output should be "welcome\[world\]

i am using regsub to add backslash "\" in place where ever i find square brackets (open or close)..
But i am not getting it... pls help out..

set a {welcome[world]}
set d [regsub -all "(?q)\[" "$a" /\[]

2 
x='welcome[world]'
echo $x | sed 's/\[\|\]/\\&/g'
3

thanks. can we do the same in tcl expect script.. pls help

4 
set a {welcome[world]}
regsub -all {\[} $a {\\[} a
regsub -all {\]} $a {\\]} d

puts $d
5

can we do it in a single line???

6 
set a {welcome[world]}
regsub -all (\\\[|\\\]) $a {\\\1} a

puts $a