[R] regexpr help (match.length=0)

[McGehee, Robert]

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?

> regexpr("[[:alpha:]]*", "123abc")
[1] 1
attr(,"match.length")
[1] 0

Thanks, Robert

> R.version
               _
platform       x86_64-apple-darwin9.8.0
arch           x86_64
os             darwin9.8.0
system         x86_64, darwin9.8.0
status         Patched
major          2
minor          11.0
year           2010
month          05
day            11
svn rev        51984
language       R
version.string R version 2.11.0 Patched (2010-05-11 r51984)