R Regular Expressions - Metacharacters

I thought that I can use metacharacters such as \w to match word characters
with one backslash.  But for some reason, I need to include two backslashes.

grepl(pattern='\w', x="what")

Error: '\w' is an unrecognized escape in character string starting "\w"

grepl(pattern='\\w', x="what")

[1] TRUE

I can't find the reason for this on the help pages.  Does anyone know why?

grepl wants a string containing a single backslash.  R uses the 
backslash as an escape character, so you need to double it in your 
source, so the string ends up containing just one.

"\w" is interpreted by R as an escaped w, which doesn't make sense.

"\\w" is interpreted by R as a backslash followed by a w, and then the 
\w is interpreted by grepl the way you want.

Duncan Murdoch