How to do case insensitive match with regexmatch in google sheets?

Not sure exactly what is $A$1, but the second parameter is still a regular expression, so I am assuming you can pass a flag like:

=filter({B:B}, regexmatch({B:B}, "(?i)in"))

if $A$1 is a string, maybe you can use the concatenate function so the output result is the flag plus your value:

=filter({B:B}, regexmatch({B:B}, CONCATENATE("(?i)", $A$1)))

You can use:

=regexmatch($I1,"(?i)tex")

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