Fix pattern for needle tag

[perlpunk]

Developer console showed me this error:

Pattern attribute value ^[A-Za-z0-9-_]+$ is not a valid regular expression:
Uncaught SyntaxError: Invalid regular expression: /^[A-Za-z0-9-_]+$/v:
Invalid character class

Apparently dashes have to be escaped. Moving it to the end of the character class (this would have fixed it for a perl regex) did not help.

Not sure if this can be tested somehow.