[R] How to remove the double or single quote from a string (unquote?)?

[JingJiang Yan]

Is there a function to get a string without a pair of quotes around it?

I have several expressions like:
glm(V12 ~ V3, family=binomial, data=df1)
glm(V12 ~ V4, family=binomial, data=df1)
...
glm(V12 ~ V8, family=binomial, data=df1)

As you can see, the only differences among them are V3 ... V8.
Because sometimes several of these expressions are performed many times,
I want to use a variable "i" to change the V3 ... V8. I did this with:

 > i <- 3:8
 > glm(V12 ~ paste("V", i, sep=""), family=binomial, data=df1)

However, it seems the paste always returns a variable name with a pair 
of quotes, which were wrong in such condition.
I only find a function "sQuote" to add quotes to a string, and it looks 
I am looking for an opposite function of it.
Any advice will be appreciated.