Formula parsing breaks if an item key contains ")", perhaps other chars as well.

• zbx-web will accept the formula on form submission.
• Next calculation run an error will occur, item status will become 'not supported'

In this example, the closing ")" of the perfmon_counter (instance) is interpreted as the closing ")" of last().
Formula:
last( host:perf_counter["\some.perfmon.context(instance)\metric"] ) + last( host:some.other.key )
Error:
Cannot evaluate function [last()]: item [host:perf_counter["\some.perfmon.context(instance] not found

I did try to add escaping & additional quoting, doesn't seem to help.