I'm pretty sure I'm missing something obvious here, but I cannot make R to use non-greedy regular expressions:
> library(stringr)
> str_match('xxx aaaab yyy', "a.*?b")
[,1]
[1,] "aaaab"
Base functions behave the same way:
> regexpr('a.*?b', 'xxx aaaab yyy')
[1] 5
attr(,"match.length")
[1] 5
attr(,"useBytes")
[1] TRUE
I would expect the match to be just ab
as per 'greedy' comment in http://stat.ethz.ch/R-manual/R-devel/library/base/html/regex.html:
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.)
Could someone please explain me what's going on?
Update. What's crazy is that in some other cases non-greedy patterns behave as expected:
> str_match('xxx <a href="abc">link</a> yyy <h1>Header</h1>', '<a.*>')
[,1]
[1,] "<a href=\"abc\">link</a> yyy <h1>Header</h1>"
> str_match('xxx <a href="abc">link</a> yyy <h1>Header</h1>', '<a.*?>')
[,1]
[1,] "<a href=\"abc\">"
Difficult concept so I'll try my best... Someone feel free to edit and explain better if it is a bit confusing.
Expressions that match your patterns are searched from left to right. Yes, all of the following strings aaaab
, aaab
, aab
, and ab
are matches to your pattern, but aaaab
being the one that starts the most to the left is the one that is returned.
So here, your non-greedy pattern is not very useful. Maybe this other example will help you understand better when a non-greedy pattern kicks in:
str_match('xxx aaaab yyy', "a.*?y")
# [,1]
# [1,] "aaaab y"
Here all of the strings aaaab y
, aaaab yy
, aaaab yyy
matched the pattern and started at the same position, but the first one was returned because of the non-greedy pattern.
So what can you do to catch that last ab
? Use this:
str_match('xxx aaaab yyy', ".*(a.*b)")
# [,1] [,2]
# [1,] "xxx aaaab" "ab"
How does it work? By adding a greedy pattern .*
in the front, you are now forcing the process to put the last possible a
into the captured group.