When writing a regex patter like [a-zA-Z0-9_-] which have - in char class,
it is better that put - at top. [-a-zA-Z0-9_]
If - is put at last and after a character: [_-],
As a parser of regexp pattern, when met - after _ (a literal character),
it can't be determined as a range operator or a literal character soon.
So parser pended it, and read next ] (end of char class),
then treat previous - as a leteral character.
It is not easy behavior.
But if - is put at top of char class,
parser easily distiguish as a literal character.
Putting - at top of regexp's char class, is kindness for parser and its efficiency.