[R] gsub regexp question

Charilaos Skiadas
2007-01-27T21:34:39Z

On Jan 27, 2007, at 3:41 PM, Phillimore, Albert wrote:

> Dear R Users,
>
> I am trying to users gsub to remove multiple cases of square
> brackets and their different contents in a character string. A
> sample of such a string is shown below. However, I am having great
> difficulty understanding regexp syntax. Any help is greatly
> appreciated.
>
> Ally
>
> "tree STATE_286000 [&lnP=-12708.453945423369] = [&R] ((((((15
> [&rate=0.009761226401396686]:7.040851727747465,17
> [&rate=0.011500289631135564]:7.040851727747465)
> [&rate=0.010986570567484494]:2.257049446900292,(18
> [&rate=0.009123432243563103]:2.461289418776003,19
> [&rate=0.00981822432115329]:2.461289418776003)"

Is this what you want? I tend to prefer perl regular expressions:

 > str <- "tree STATE_286000 [&lnP=-12708.453945423369] =
[&R]
((((((15[&rate=0.009761226401396686]:7.040851727747465,17
[&rate=0.011500289631135564]:7.040851727747465)
[&rate=0.010986570567484494]:2.257049446900292,(18
[&rate=0.009123432243563103]:2.461289418776003,19
[&rate=0.00981822432115329]:2.461289418776003)"
 > gsub("\\[[^\\]]+\\]","",str, perl=T)
[1] "tree STATE_286000  =
((((((15:7.040851727747465,17:7.040851727747465):2.257049446900292,
(18:2.461289418776003,19:2.461289418776003)"


As an explanation, \\[ and \\] match the two square brackets you
want. We need to escape the brackets with the backslashes because
they have a special meaning in perl regular expressions.

In perl regexps, "[....]" stands for "match a single character
that
is like what we have in the .... For instance [ab] will match an a or
a b. [a-z] will match all lowercase characters. A ^ as a first
character in there means "match all but what follows". for instance
[^a-z] means match anything but lowercase characters. So [^\\]] means
match any character but a closing bracket.

Finally the plus sign afterwards means: match at least one. So [^\\]]
+ means "match any sequence of characters that does not contain a
closing bracket. So the whole thing now matches an opening bracket,
followed by all characters until a corresponding closing bracket.
This will not work if you have nested pairs of brackets, [like [so]].
That is a tad more delicate, and we can discuss it if you really need
to deal with it.

Haris