There could be an warning dialog after clicking [OK]
IF “Regular Expressions Match New Line Characters” is checked
AND “Additional Lines to Search for Regular Expressions” is set to ‘0’

“You have chosen to use RegEx in multi line mode.
Please note that you have to set the roughly expected “Additional Lines” option too.”

Or an better idea:
That two options could be somehow “grouped” by an thin frame line,
and the “Additional Lines” option could be greyed-out
while “Match New Line” is disabled,
to make the relationship better visible.

“[X] Regular Expressions Match New Line Characters”
          “Multi Line mode search max. for [ 0 ] additional lines.”

– – –

Or maybe these option could be set to an higher amount as ‘0’ by default.
Would be an amount of, lets say, ’30’ really that slow?