Fix escaping in regex and add tests

[MartinRiese]

Summary

Fix regex issue found be automated scan. I doubt that code path has been used

https://dimagi.atlassian.net/browse/USH-5176

Feature Flag

None

Product Description

Safety Assurance

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change
• I have confidence that this PR will not introduce a regression for the reasons below

Automated test coverage

./test "updateInstanceQuery"

QA Plan

Safety story

I am pretty sure this code path has not been used in a while, since it is not working as expected. See first of the tests.

Rollback instructions

• This PR can be reverted after deploy with no further considerations