[R] grep/regexp

[David Winsemius]

> On Dec 1, 2015, at 2:47 PM, Ravi Varadhan <ravi.varadhan at jhu.edu> wrote:
> 
> Hi,
> I would appreciate some help with using grep().  I have a bunch of variables in a data frame and I would like to select some of them using grep.  Here is an example of what I am trying to do:
> 
> vars <- c("Fr_I_total", "Fr_I_percent_of_CD4", "Ki.67_in_Fr_I_percent_of_Fr_I", "Fr_II_percent_of_CD4", "Ki.67_in_Fr_II_percent_of_Fr_II")
> 
>> From the above vector, I would like to select those variables beginning with `Fr' and containing `percent' in them.   In other words, I would like to get the variables "Fr_I_percent_of_CD4" and "Fr_II_percent_of_CD4".
> 
> How can I use grep() to do this?

> grep("^Fr.*percent", vars, value=TRUE)
[1] "Fr_I_percent_of_CD4"  “Fr_II_percent_of_CD4"


> More generally, are there any good online resources with examples like this for the use of grep() and regexp() in R?  I didn't find the help pages for these very user-friendly.
> 

There are several interactive regex websites where you can get commented and tested solutions. They have the disadvantage that they don’t use the extra backslashes that R requires since both R and regex use backslashes as escape characters.

I learned regex from the R's ?regex page and by watching Gabor Grothendeick’s postings to Rhelp. (I’ve probably gone over the ?regex page 20 or thirty times.)

These days there are a great many regex questions and answers on Stack Overflow , many of them again written by Maestro Grothendeick.

— 

David Winsemius
Alameda, CA, USA