When searching for a regular expression, you can click the Dot button on the search toolbar to turn on the "dot matches newline" search option. This option makes the dot match any character, including line break characters.
Turning on this option is the same as putting the mode modifier (?s) at the start of your regular expression. If you do use the mode modifier (?s) or (?-s) then the mode modifier overrides the state of the checkbox.
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Search Option: Regular Expression