I assume you want $1, $2, $3 and $4 to be something other than "." and "-". If so, how about: 
perl -le'
    $_ = "..."; 
    s/[\.-]+//g;
    if (/(\S)(\S)(\S)(\S)$/) {
        print "$1$2$3$4"
    }
'

[download]

or just 

perl -le'
    $_ = "..."; 
    s/[\.-]+//g;
    print substr $_, -4;
'

[download]

In reply to Re: why this regular expression is so slow? by ikegami
in thread why this regular expression is so slow? by fnever

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