In a regular expression, non-ASCII and non-printable characters other than newline are not special, and represent themselves. For example, in a locale using UTF-8 the command ‘grep 'Λ ω'’ (where the white space between ‘Λ’ and the ‘ω’ is a tab character) searches for ‘Λ’ (Unicode character U+039B GREEK CAPITAL LETTER LAMBDA), followed by a tab (U+0009 TAB), followed by ‘ω’ (U+03C9 GREEK SMALL LETTER OMEGA).
Suppose you want to limit your pattern to only printable characters
(or even only printable ASCII characters) to keep your script readable
or portable, but you also want to match specific non-ASCII or non-null
non-printable characters. If you are using the -P
(--perl-regexp) option, PCREs give you several ways to do
this. Otherwise, if you are using Bash, the GNU project’s shell, you
can represent these characters via ANSI-C quoting. For example, the
Bash commands ‘grep $'Λ\tω'’ and ‘grep $'\u039B\t\u03C9'’
both search for the same three-character string ‘Λ ω’
mentioned earlier. However, because Bash translates ANSI-C quoting
before grep sees the pattern, this technique should not be
used to match printable ASCII characters; for example, ‘grep
$'\u005E'’ is equivalent to ‘grep '^'’ and matches any line, not
just lines containing the character ‘^’ (U+005E CIRCUMFLEX
ACCENT).
Since PCREs and ANSI-C quoting are GNU extensions to POSIX, portable shell scripts written in ASCII should use other methods to match specific non-ASCII characters. For example, in a UTF-8 locale the command ‘grep "$(printf '\316\233\t\317\211\n')"’ is a portable albeit hard-to-read alternative to Bash’s ‘grep $'Λ\tω'’. However, none of these techniques will let you put a null character directly into a command-line pattern; null characters can appear only in a pattern specified via the -f (--file) option.