Question

Possible Duplicate:
Extract info inside all parenthesis in R (regex)

I have a string

df

Peoplesoft(id-1290)

I like to capture characters between the parentesis, for example. I like to get id-1290 from the above example.

I used this:

x <- regexpr("\\((.*)\\)", df) 

this is giving me numbers like

[1] 10

Is there an easy way to grab text between parentesis using regex in R?

Answer 1

I prefer to use gsub() for this:

gsub(".*\\((.*)\\).*", "\\1", df)
[1] "id-1290"

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The regex works like this:

• Find text inside the parentheses – not your real parentheses, but my extra set of parentheses, i.e. (.*)
• Return this as a back-reference, \\1

In other words, substitute all text in the string with the back reference

---

If you want to use regexp rather than gsub, then do this:

x <- regexpr("\\((.*)\\)", df)
x

[1] 11
attr(,"match.length")
[1] 9
attr(,"useBytes")
[1] TRUE

This returns a value of 11, i.e. the starting position of the found expression. And note the attribute match.length that indicates how many characters were matched.

You can extract this with attr:

attr(x, "match.length")
[1] 9

And then use substring to extract the characters:

substring(df, x+1, x+attr(x, "match.length")-2)
[1] "id-1290"