Long string not producing any search results

Hey,

So, I have hundreds of old posts on my blog that all end with the same string, which is a call to action to sign up via RSS or email. (The string is within the body of the posts.) Those links are obsolete, so I’d like to remove the string from all of those posts. All of the strings look just like this, or something very similar (I have a record of the variants):

<strong><em>Enjoy this post? Subscribe to Great White Snark by </em><a onclick="javascript:urchinTracker ('/outbound/article/www.feedburner.com');" href="http://www.feedburner.com/fb/a/emailverifySubmit?feedId=769701&loc=en_US"><em>email</em></a><em> or by </em><a onclick="javascript:urchinTracker ('/outbound/article/feeds.feedburner.com');" href="http://feeds.feedburner.com/greatwhitesnark/yqzr" target="_blank"><em>RSS</em></a><em>.</em></strong>

It’s a long string. Even though I know it exists in the body of many of my posts, Search Regex says no instances were found when I try to search it. (I had a similar problem with another search-and-replace plugin.) If I break the string into shorter chunks (e.g., just searching on the link to Feedburner), the plugin is able to locate the hundreds of instances. Any suggestions re: what I could do to help the plugin successfully locate these long strings for me?

Thanks in advance for the help!

https://wordpress.org/plugins/search-regex/