[R] grep for multiple pattern?

[Marc Schwartz]

On Feb 13, 2014, at 8:43 AM, Rainer M Krug <Rainer at krugs.de> wrote:

> Hi
> 
> I want to search for multiple pattern as grep is doing for a single
> pattern, but this obviously not work:
> 
>> grep("an", month.name)
> [1] 1
>> grep("em", month.name)
> [1]  9 11 12
>> grep("eb", month.name)
> [1] 2
>> grep(c("an", "em", "eb"), month.name)
> [1] 1
> Warning message:
> In grep(c("an", "em", "eb"), month.name) :
>  argument 'pattern' has length > 1 and only the first element will be used
>> 
> 
> Is there an equivalent which returns the positions as grep is doing, but
> not using the strict full-string matching of match()?
> 
> I could obviously do:
> 
>> unlist( sapply(pat, grep, month.name ) )
> an em1 em2 em3  eb
>  1   9  11  12   2
> 
> but is there a more compact command I am missing?
> 
> Thanks,
> 
> Rainer


The vertical bar '|' acts as a logical 'or' operator in regex expressions:

> grep("an|em|eb", month.name)
[1]  1  2  9 11 12

> grep("an|em|eb", month.name, value = TRUE)
[1] "January"   "February"  "September" "November"  "December" 


Regards,

Marc Schwartz