How to escape “<” character in regex in Solr schema.xml?

[smooth almonds]

Using Solr 3.5.0 and in my schema.xml I'm using the following to mark the end
of sentences and replace the end punctuation with a symbolic token:

<charFilter class=&quot;solr.PatternReplaceCharFilterFactory&quot;
pattern=&quot;(?&lt;=[^.!?\\s][^.!?]*(?:[.!?](?![']?\s|$)[^.!?]*)*)[.!?]+(?=\\s|$)&quot;
replacement=&quot; monkeysentence&quot;/>

I'm not sure if that will even work for what I want, but first I need to
solve the problem of escaping the '<' character in the first '?<='
lookbehind.

I get the following error:

org.xml.sax.SAXParseException: The value of attribute "pattern" associated
with an element type "null" must not contain the '<' character.

I've tried using a '\' as in:

pattern="(?\<=[^.!?\\s][^.!?]*(?:[.!?](?![']?\s|$)[^.!?]*)*)[.!?]+(?=\\s|$)"

But I get the same error.

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