regular expression with space

I am using regular expression in R with the following code:

> temp <- c("Herniorrhaphy, left inguinal", "Herniorrhaphy, right inguinal")
> grep("Herniorrhaphy, [left|right] inguinal",temp)
integer(0)
> grep("Herniorrhaphy, [left inguinal|right inguinal]",temp)
[1] 1 2

I wonder why the two regular expressio开发者_如何转开发n give difference result, thanks.

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According to regexp explanation in the documentation (http://stat.ethz.ch/R-manual/R-devel/library/base/html/regex.html):

Note that alternation does not work inside character classes, where | has its literal meaning.

That explains why the first alternative doesn't return any results because '[' and ']' characters denote a character class. The correct sytax should be:

grep("Herniorrhaphy, (left|right) inguinal",temp)

On my R, the second alternative also returns empty set as well:

> temp <- c("Herniorrhaphy, left inguinal", "Herniorrhaphy, right inguinal")
> grep("Herniorrhaphy, [left inguinal|right inguinal] inguinal",temp)
integer(0)
> 

Are you sure you are copying directly from the workspace?

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I think you want brackets ( ) not character class [ ], ie

"Herniorrhaphy, (left|right) inguinal"
"Herniorrhaphy, (left inguinal|right inguinal)"