gistfile1.txt

--- possible input :

key [t|a]
<style type="text/css"><!--/*--><![CDATA[/*><!--*/
key2[param] [u|1]
something else
key3[[param,param],param] [z|44]

--- desired output (validating trimmed $1 for something like [a-zA-Z0-9\[*] ):

key
key2[
key3[

--- reasoning :

key [t|a]                         <- [ present, no [ in $1, whole $1 taken
<style <![CDATA[/*><!--*/         <- $1 does not match
key2[param] [u|1]                 <- [ present on line and in $1, $1[ taken
something else                    <- no [ on the whole line
key3[[param,param],param] [z|44]