From ?regexp, in the Extended Regular Expressions section:
The caret ‘^’ and the dollar sign ‘$’ are metacharacters that
respectively match the empty string at the beginning and end of a
line. The symbols ‘\<’ and ‘>’ match the empty string at the
beginning and end of a word. The symbol ‘\b’ matches the empty
string at either edge of a word, and ‘\B’ matches the empty string
provided it is not at an edge of a word. (The interpretation of
‘word’ depends on the locale and implementation: these are all
extensions.)
From Perl-like Regular Expressions:
The escape sequences ‘\d’, ‘\s’ and ‘\w’ represent any decimal
digit, space character and ‘word’ character (letter, digit or
underscore in the current locale: in UTF-8 mode only ASCII letters
and digits are considered) respectively, and their upper-case
versions represent their negation. Vertical tab was not regarded
as a space character in a ‘C’ locale before PCRE 8.34 (included in
R 3.0.3). Sequences ‘\h’, ‘\v’, ‘\H’ and ‘\V’ match horizontal
and vertical space or the negation. (In UTF-8 mode, these do
match non-ASCII Unicode code points.)
Note that backslashes usually need to be doubled/protected in R input, e.g. you would use "\\h" to match horizontal space.
From ?Quotes:
Backslash is used to start an escape sequence inside character
constants. Escaping a character not in the following table is an
error.
\n newline
\r carriage return
\t tab
As others comment above, you may need a little more help if you're getting started with regular expressions for the first time. This is a little bit off-topic for StackOverflow (links to off-site resources), but there are some links to regular expression resources at the bottom of the gsubfn package overview. Or Google "regular expression tutorial" ...
