[R] on gsub (simple, but not to me!) sintax

[David Winsemius]

On Nov 16, 2009, at 8:21 AM, Ottorino-Luca Pantani wrote:

> Dear R users,
> my problem today deals with my ignorance on regular expressions.
> a matter I recently discovered.
>
> Consider the following
>
> foo <-
> c("V_7_101110_V",  "V_7_101110_V",  "V_9_101110_V",  "V_9_101110_V",
> "V_9_s101110_V",  "V_9_101110_V",  "V_9_101110_V",  "V_11_101110_V",
> "V_11_101110_V", "V_11_101110_V", "V_11_101110_V", "V_11_101110_V",
> "V_17_101110_V", "V_17_101110_V")
>
> what I'm trying to obtain is to add a zero in front of numbers below  
> 10,
> as in
>
> c("V_07_101110_V",  "V_07_101110_V",  "V_09_101110_V",   
> "V_09_101110_V",
> "V_09_101110_V",  "V_09_101110_V",  "V_09_101110_V",  "V_11_101110_V",
> "V_11_101110_V", "V_11_101110_V", "V_11_101110_V", "V_11_101110_V",
> "V_17_101110_V", "V_17_101110_V")
>
Any of these (the need for doubling of the "\\" for the back-reference  
seems to be the main issue:
 > gsub("_([[:digit:]])_.", "_0\\1_", foo)
  [1] "V_07_01110_V"  "V_07_01110_V"  "V_09_01110_V"  "V_09_01110_V"   
"V_09_101110_V"
  [6] "V_09_01110_V"  "V_09_01110_V"  "V_11_101110_V" "V_11_101110_V"  
"V_11_101110_V"
[11] "V_11_101110_V" "V_11_101110_V" "V_17_101110_V" "V_17_101110_V"

 > gsub("_(\\d)_.", "_0\\1_", foo)
  [1] "V_07_01110_V"  "V_07_01110_V"  "V_09_01110_V"  "V_09_01110_V"   
"V_09_101110_V"
  [6] "V_09_01110_V"  "V_09_01110_V"  "V_11_101110_V" "V_11_101110_V"  
"V_11_101110_V"
[11] "V_11_101110_V" "V_11_101110_V" "V_17_101110_V" "V_17_101110_V"

 > gsub("V_(.)_", "V_0\\1_", foo)
  [1] "V_07_101110_V"  "V_07_101110_V"  "V_09_101110_V"   
"V_09_101110_V"  "V_09_s101110_V"
  [6] "V_09_101110_V"  "V_09_101110_V"  "V_11_101110_V"   
"V_11_101110_V"  "V_11_101110_V"
[11] "V_11_101110_V"  "V_11_101110_V"  "V_17_101110_V"  "V_17_101110_V"
>
> I'm able to do this on the emacs buffer through query-replace-regexp
>
> C-M-%
> search for
> V_\(.\)_
> and substitute with
> V_0\1_
>
> but I completely ignore how to do it with gsub within R
> and the help is quite complicate to understand
> (at least to me, at this moment in time)
>
> I can search the vector through
> grep("V_._",  foo)
>
> but I always get errors either on
> gsub('V_\(.\)_', 'V_0\1_', foo)
>
>
> or I get not what I'm looking for on
> gsub('V_._', 'V_0._', foo)
> gsub('V_._', 'V_0\1_', foo)
>
> Thanks in advance
> -- 
> Ottorino-Luca Pantani, Università di Firenze
> Dip. Scienza del Suolo e Nutrizione della Pianta
> P.zle Cascine 28 50144 Firenze Italia
> Ubuntu 8.04.3 LTS -- GNU Emacs 23.0.60.1 (x86_64-pc-linux-gnu, GTK+  
> Version 2.12.9)
> ESS version 5.5 -- R 2.10.0
>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT