linux bash man page里面提到：
ANSI-C Quoting
Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specifed by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:
\a - alert (bell)
\b - backspace
\e - an escape character (not ANSI C)
\f - form feed
\n - newline
\r - carriage return
\t - horizontal tab
\v - vertical tab
\\ - backslash
\' - single quote
\nnn - the character whose
ASCII code is the octal value nnn (one to three digits) \xnnn - the character whose
ASCII code is the hexadecimal value nnn (one to three digits)
The expanded result is single-quoted, as if the dollar sign had not been present.
也就是说如果你想在source.txt里面grep匹配tab+targetword内容，那么写法应该是：
grep $'\t'targetword source.txt.