Extract a numeric pattern between two only underscores in string

Question

I am relatively new to regular expressions and I am running into a dead end. I have a data frame with a column that looks like this:

year1
GMM14_2000_NGVA
GMM14_2001_NGVA
GMM14_2002_NGVA
...
GMM14_2014_NGVA

I am trying to extract the year in the middle of the string (2000,2001, etc). This is my code thus far

gsub("[^0-9]","",year1))

Which returns the number but it also returns the 14 that is part of the string:

142000
142001

Any idea on how to exclude the 14 from the pattern or how to extract the year information more efficiently?

Thanks

Answer 1

Use the following gsub:

s  = "GMM14_2002_NGVA"
gsub("^[^_]*_|_[^_]*$", "", s)

See IDEONE demo

The regex breakdown:

Match...

• ^[^_]*_ - 0 or more characters other than _ from the start of string and a_
• | - or...
• _[^_]*$ - a _ and 0 or more characters other than _ to the end of string

and remove them.

As an alternative,

library(stringr)
str_extract(s,"(?<=_)\\d{4}(?=_)")

Where the Perl-like regex matches 4-digit substring that is enclosed with underscores.