McGehee, Robert wrote:
R-help,
Sorry if this is more of a regex question than an R question. However,
help would be appreciated on my use of the regexpr function.
In the first example below, I ask for all characters (a-z) in 'abc123';
regexpr returns a 3-character match beginning at the first character.
regexpr("[[:alpha:]]*", "abc123")
[1] 1
attr(,"match.length")
[1] 3
However, when the text is flipped regexpr, and I ask for a match of all
characters in '123abc', regexpr returns a zero-character match beginning
at the first character. Can someone explain what a zero length match
means (i.e. why not return -1), and why the result isn't 4,
match.length=3?
It means it matches 0 characters, which is fine since you use *, which
means match 0 or more occurrences of the regex. It sounds like you want
+ instead of *. Also see gregexpr.
regexpr("[[:alpha:]]*", "123abc")
[1] 1
attr(,"match.length")
[1] 0
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