R why is gregexpr not returning all occurrences of a regex pattern?

Question

I have the following string:

FFDEFFDFFDCFFDDFFAAF

and the following regex pattern:

FF..FF

Running gregexpr will result in the following:

gregexpr('FF..FF','FFDEFFDFFDCFFDDFFAAF')

[[1]]
[1] 1 8
attr(,"match.length")
[1] 6 6
attr(,"useBytes")
[1] TRUE

But there is one match missing, as there are three occurrences of the pattern:

FFDEFFDFFDCFFDDFFAAF
FF..FF ||  ||  || 
       FF..FF  ||
           FF..FF

Any idea why is this happening and how to resolve this?

Regex doesn't do overlapping patterns, [see here](http://stackoverflow.com/q/320448/903061) [or here](http://stackoverflow.com/q/11430863/903061) for ways to get around this with lookahead. — Gregor Thomas, Jun 17 '15 at 23:23

score 3 · Accepted Answer · answered Jun 17 '15 at 23:30

3

you need a look ahead expression which you can do with a perl regex:

gregexpr("(?=FF..FF)", "FFDEFFDFFDCFFDDFFAAF", perl=TRUE)

answered Jun 17 '15 at 23:30

cr1msonB1ade

Thanks, that was what i was missing after Gregor comment... The perl=TRUE – SwatchPuppy Jun 17 '15 at 23:34

1 Answers1