</?(?:(?!pb)[^>])*>

<          # "<"
/?         # optional "/"
(?:        # don't capture group
  (?!      # a position not followed by...
    pb    # "p" and a word bounday
  )
  [^>]*    # any char but ">"
)*         # as often as possible
>          # ">"

<p class="foo" title="unusual < title">
                              ---------

var html = '<img src=""><p>content</p><span style="color: red">content</span>';
html.replace(/<(?!s*/?s*pb)[^>]*>/gi,'')