How to replace “=” with “_” in vim using by Regex?

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Can you advise me a Regex to replace "-" to "_" in my html file but focus on "href" and "src" tag only. because i don't need to make any effects to contents in my webpage.e.g.the original:\[code\]<a href="http://stackoverflow.com/questions/15539307/my-profile.html">PLF Series</a><img src="http://stackoverflow.com/questions/15539307/images/my-sample.jpg">\[/code\]after modified:\[code\]<a href="http://stackoverflow.com/questions/15539307/my_profile.html">PLF Series</a><img src="http://stackoverflow.com/questions/15539307/images/my_sample.jpg">\[/code\]