The docs for ?regexp say this: "By default repetition is greedy, so the
maximal possible number of repeats is used. This can be changed to
?minimal? by appending ? to the quantifier. (There are further
quantifiers that allow approximate matching: see the TRE documentation.)"
I want the minimal match, but I don't seem to be getting it. For example,
x <- "abaca"
grep("a.*?a", x, value = TRUE)
#> [1] "abaca"
Shouldn't I have gotten "aba", which is the first match to
"a.*a"? If
not, what would be the regexp that would give me the first match to
"a.*a", without greedy expansion of the .*?
Duncan Murdoch
grep(value = TRUE) just returns the strings which match the pattern. You
have to use regexpr() or gregexpr() if you want to know where the matches
are:
```
x <- "abaca"
# extract only the first match with regexpr()
m <- regexpr("a.*?a", x)
regmatches(x, m)
# or
# extract every match with gregexpr()
m <- gregexpr("a.*?a", x)
regmatches(x, m)
```
You could also use sub() to remove the rest of the string:
`sub("^.*(a.*?a).*$", "\\1", x)`
keeping only the match within the parenthesis.
On Wed, Jan 25, 2023, 19:19 Duncan Murdoch <murdoch.duncan at gmail.com>
wrote:
> The docs for ?regexp say this: "By default repetition is greedy, so
the
> maximal possible number of repeats is used. This can be changed to
> ?minimal? by appending ? to the quantifier. (There are further
> quantifiers that allow approximate matching: see the TRE
documentation.)"
>
> I want the minimal match, but I don't seem to be getting it. For
example,
>
> x <- "abaca"
> grep("a.*?a", x, value = TRUE)
> #> [1] "abaca"
>
> Shouldn't I have gotten "aba", which is the first match to
"a.*a"? If
> not, what would be the regexp that would give me the first match to
> "a.*a", without greedy expansion of the .*?
>
> Duncan Murdoch
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
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On 25/01/2023 7:19 p.m., Duncan Murdoch wrote:> The docs for ?regexp say this: "By default repetition is greedy, so the > maximal possible number of repeats is used. This can be changed to > ?minimal? by appending ? to the quantifier. (There are further > quantifiers that allow approximate matching: see the TRE documentation.)" > > I want the minimal match, but I don't seem to be getting it. For example, > > x <- "abaca" > grep("a.*?a", x, value = TRUE) > #> [1] "abaca" > > Shouldn't I have gotten "aba", which is the first match to "a.*a"? If > not, what would be the regexp that would give me the first match to > "a.*a", without greedy expansion of the .*?Sorry, that was a dumb question. Of course grep returned the whole thing. I should be using regexpr() or some related function to extract the match. Duncan Murdoch
Perhaps sub( "^.*(a.*?a).*$", "\\1", x ) On January 25, 2023 4:19:01 PM PST, Duncan Murdoch <murdoch.duncan at gmail.com> wrote:>The docs for ?regexp say this: "By default repetition is greedy, so the maximal possible number of repeats is used. This can be changed to ?minimal? by appending ? to the quantifier. (There are further quantifiers that allow approximate matching: see the TRE documentation.)" > >I want the minimal match, but I don't seem to be getting it. For example, > >x <- "abaca" >grep("a.*?a", x, value = TRUE) >#> [1] "abaca" > >Shouldn't I have gotten "aba", which is the first match to "a.*a"? If not, what would be the regexp that would give me the first match to "a.*a", without greedy expansion of the .*? > >Duncan Murdoch > >______________________________________________ >R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see >https://stat.ethz.ch/mailman/listinfo/r-help >PLEASE do read the posting guide http://www.R-project.org/posting-guide.html >and provide commented, minimal, self-contained, reproducible code.-- Sent from my phone. Please excuse my brevity.