Update: I used a form which was needless complicated in this particular instance. I struke the related content, even though it's not technically wrong. In exchange, I added the new form. The new content is enclosed by bold parens.

Things to know:

• /$re1/ && /$re2/
  can be written as
  /^(?=.*$re1)(?=.*$re2)/s
  or as
  /^(?=.*$re1).*$re2/s

• (?:(?!$re).)
  is to regexps as
  [^$chars]
  is to characters.

  Keep in mind
  'abc' =~ /[^a]/s matches at pos 1
  'abc' =~ /^[^a]*$/s does not match
  'abc' =~ /(?:(?!ab).)/s matches at pos 1
  'abc' =~ /^(?:(?!ab).)*$/s does not match

• (
  Similarly
  !/$re1/s && /$re2/s
  can be written as
  /^(?!.*$re1)(?=.*$re2)/s
  or as
  /^(?!.*$re1).*$re2/s
  )

Any of the following do what you want

• /bar/ && !/foo/
• /bar/ && /^(?:(?!foo).)*$/s
• /^(?=.*bar)(?=(?:(?!foo).)*$)/s
• /^(?=.*bar)(?:(?!foo).)*$/s
• /^(?=(?:(?!foo).)*$).*bar/s
• ( /^(?=.*bar)(?!.*foo)/ )
• ( /^(?!.*foo)(?=.*bar)/ )
• ( /^(?!.*foo).*bar/ )