[R] gsub regexp question

Charilaos Skiadas skiadas at hanover.edu
Sat Jan 27 22:34:39 CET 2007

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]  
 > gsub("\\[[^\\]]+\\]","",str, perl=T)
[1] "tree STATE_286000  =   

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.


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