perl -e 'if ("26-04"=~/\d{2}[\-._]\d{2}([\-._]\d{2})*/) { print "yes\n
+" }'

#prints
yes
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Seems to work. No need to escape the '-' as it would not be used as range character when it is the first character in a character class

PS: Use <c>-tags around your regex so that it gets displayed correctly in the browser