Stupid regex question

[Seth Gordon]

Josh ChaitinPollak wrote:
> I'm trying to detect the presence of a word on a line using sed. Is  
> there a way to say something like, 'if the match fails, don't return  
> anything'? For example:
> 
> pardsbane $ echo "this is a -p param test" | sed -e 's/.*\-p \([a-z]* 
> \).*/\1/'
> param
> pardsbane $ echo "this is a param test" | sed -e 's/.*\-p \([a-z]* 
> \).*/\1/'
> this is a param test

$ echo "this is a param test" | sed -e 's/.*\-p \([a-z]* \).*/\1/ ; t ; 
d' | sed -e 's/^\-p //'
$ echo "this is a -p param test" | sed -e 's/.*\-p \([a-z]* \).*/\1/ ; t 
; d' | sed -e 's/^\-p //'
param

("t" means "if the last regexp matched, branch to the end of the 
script"; "d" means "delete the whole pattern space")