R help - Dec 2015 - grep/regexp

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?

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.

Thank you very much,
Ravi

Ravi Varadhan, Ph.D. (Biostatistics), Ph.D. (Environmental Engg)
Associate Professor,  Department of Oncology
Division of Biostatistics & Bionformatics
Sidney Kimmel Comprehensive Cancer Center
Johns Hopkins University
550 N. Broadway, Suite 1111-E
Baltimore, MD 21205
410-502-2619


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> 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