I have two possible cases in some XML.

Case 1, incorrect: <tag att1="text1"/>

Case 2, correct: <tag att1="text1" att2="text2"/>

So I am trying to find all examples of <tag att1="text1"/> to add attribute 2.

I want to find all my examples of case 1 and add the string for case 2.

<tag att1="(.*?)"/> find both case 1 and case 2

find

<tag att1="(.*?)"(?! att2="(.*?)"/>)

is failing because att1=(.*?) because it still selects att2

<tag att1=(".*?")(?! att2=".*?"/>)

is failing because att1=(.*?) because it still selects att2

How do I prevent my search from finding my case 2 xml?