echo a | grep -i --color '\a' echo a | grep -i --color '\A'
Of the above two commands, only the second colorizes the printed "a". (An old GNU grep on NetBSD (grep 2.5.1a nb1) does the opposite of a modern grep: it will show color for \a, but none for \A.) Bug was found in GNU grep 3.1 while checking whether it understands \d as a shorthand for [0-9]. Still present in 3.4. The locale does not appear to matter. In a report against glibc [1] that seems to be related, a comment says that "Unknown backslash escapes invoke undefined behaviour." But where in the documentation does it say so? When searching for a regular expression in GNU nano (^W M-R), nano says it cannot find any \a. But for \A, it will find all "a"s and "A"s. Nano's default search is case insensitive. See [2] for the original report by Ben Addis. I suppose this is the same or a related bug in the regex module of gnulib, which nano uses. [1] https://sourceware.org/bugzilla/show_bug.cgi?id=22425 [2] https://savannah.gnu.org/bugs/?57852 Benno
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