I have existing scenario files that are modeled directly out of the documentation:
https://sipp.readthedocs.io/en/latest/scenarios/variables.html?highlight=search_in#variables

<recv request="INVITE">
  <action>
    <ereg regexp="<sip:([^;@]*)" search_in="hdr" header="To:" assign_to="dummy,uri" />
  </action>
</recv>
<Reference variables="dummy" />

The less than character < inside the quotes of the regexp tag (regexp="<sip:([^;@]*)") causes the following failure when running with any release after v3.6.0:
2024-04-20 12:55:46.344095 1713635746.344095: Invalid XML in scenario

Existing scenario files work fine up until v3.5.3. They will not pass the XML check logic starting with v3.6.0.

I'm pretty sure this commit introduced the regression:
411ccf4 @wdoekes

It might be the deletion of this function call as the xp_find_start_tag_end() function is checking for embedded strings in quotes "".