Comment by masklinn

It’s not start of line though, unless the engine is in multiline mode. Here is the documentation for Python’s re for instance:

> Matches the start of the string, and in MULTILINE mode also matches immediately after each newline.

Or JavaScript:

> An input boundary is the start or end of the string; or, if the m flag is set, the start or end of a line.

\A and \Z are start/end of input regardless of mode… when they’re available, that’s not the case of all engines.