if($line =~ /Handling NSPAdvice for mechanism [4]/) {
    # the RE above would match "Handling NSPAdvice for mechanism 4"

##</code><code>##

    if($line =~ /Handling NSPAdvice for mechanism \[4\]/) {
    # alternative: if ( index($line,'Handling NSPAdvice for mechanism [4]')>=0 ) {

##</code><code>##

            if ($line =~ /Authentication mechanism returned [[](\S*)[]]/) {

##</code><code>##

            if ($line =~ /Authentication mechanism returned \[(\-?\d+)\]/) {
            # the RE above would match e.g. [4] -> $1=4,  [-10] -> $1=-10, etc.