Josef Eschgfaeller wrote:
Why sometimes one has to put a double
backslash in regular expressions, but
often simple backslashes work too?
Is only a \ required for giving a
metacharacter its usual meaning?
The general reason is that both R and grep use \ as an escape character.
If you want to send an escape to grep you need to escape the escape in R.
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u=grep('\\{[\\-u]x',a,perl=T)
# equivalent to
u=grep('\{[\-u]x',a,perl=T)
No. The first one passes the string containing "\{[\-u]x" to grep
(after processing the escapes). The second one passes "{[-u]x" to grep.
# but
u=grep('\w',a,perl=T)
\w has no special meaning in R, so this pattern becomes "w".
# is not correct and requires
u=grep('\\w',a,perl=T)
This is the pattern "\w". I don't know if this has special meaning to
pcre, but I guess it must, or you wouldn't have tried it.
Duncan Murdoch
---------------------------------------
Josef Eschgf�ller
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