You have several options. Forbid spaces. Forbid double quote marks. Remove the closing angle bracket in your regular expression.

In the interests of efficiency and efficacy, merlyn has written link verifiers. See his homepage. You might also have better luck with a module such as HTML::Parser or HTML::TokeParser, which can provide the 'href' attribute of an 'a' tag.

For reading this far, here's a regex I'd try for a quick and dirty solution:

$buf =~ /<a href="([^"])"/gis