Single-line and Multiline
In the Modifiers flyout are two sets of modifiers, Single-line On/Off and Multiline On/Off. These modifiers influence the behavior of the . wildcard (“match everything”) and the location markers ^ (beginning of paragraph) and $ (paragraph end).
Single-line mode determines the behavior of . (the dot wildcard). When single-line mode is off, which is GREP’s default state, .* matches everything from the cursor position to the end of the paragraph, except the paragraph mark itself. Enable single-line mode (with (?s)), and .* ignores paragraph marks, selecting everything from the insertion point to the end of the story. If there is no insertion point and you have a text frame selected, (?s).* selects the whole story—not just the text in the selected text frame—including any overset text.
The single-line/multiline settings influence the behavior of ^ and $. When multiline is enabled, which is the default state, ^ and $ match the beginning and the end of each paragraph, respectively; they respect paragraph boundaries. If multiline is disabled with (?-m), ^ and $ ignore the paragraph boundaries and become in effect beginning-of-story and end-of-story markers—synonymous with \A and \z.