True allergies to cow's milk are relatively uncommon. Only 2 to 3 percent of children are allergic to milk, according to the AAP, and almost all of them outgrow it by age 3. (Learn the difference between a milk allergy and lactose intolerance .).
The underscore [ _ ] (also called underline, underbar, low line, or low dash) is a character that originally appeared on the typewriter and was primarily used to underline words.
The \w means any word character which usually means alphanumeric (letters, numbers, regardless of case) plus underscore (_) The ^ anchors to the beginning of a string, and the $ anchors To the end of a string, which means that, in this case, the match must start at the beginning of a string and end at the end of the string.