the characters -~ in the character class

Unless the hyphen is the very first character, it signifies a range. In this case, it's the range ' ' (space) through '~' (tilde) - which is essentially the entire range of printable ASCII characters.

So what this test is saying: is there a "line" of "text" - where "line" is defined by the ^ and $ anchors, and "text" is defined as "all the printable ASCII characters, plus the selected whitespace characters newline, carriage return, tab, and space."