How to match word surronded by spaces OR at the end / beginning of the string using Perl regexp?

I want to find sequences matching my regexp should they be in the middle of the string surrounded by spaces, in the end or beginning or be the only thing in a string.

Example: Let's assume the sequence 'qwe45rty' is what we are looking for. I want to be able to get positive on all of these strings:

'qwe45rty' 'qwe45rty blabla' 'smth qwe45rty blabla' 'smth qwe45rty' ' qwe45rty '

But none of these:

'aaqwe45rty' 'qwe45rtybb' 'aaqwe45rtybb'

Best what I came up with is smth like this:

if ( ($a =~ /\s+$re\s+/) or
     ($a =~ /^$re\s+/)   or
     ($a =~ /\s+$re$/)   or
     ($a =~ /^$re$/)        )
{
    # do stuff
}

which can't be the best way to do that :)

Any suggestions?