How to extract substring between patterns "_" and "." in R [duplicate]

Question

I have many filenames which look like:

txt= "MA0051_IRF2.xml"

I want to extract IRF2 which is between "_" and ".". How do I do this in R?

Answer 1

To achieve this, you need a regexp that

• matches an (optional) arbitrary string in front of the _ : .*
• matches a literal _ : [_]
• matches everything up to (but not including) the next . and stores it in capturing group no. 1 : ([^.]+)
• matches a literal . : [.]
• matches an (optional) arbitrary string after the . : .*

In your call to gsub, you then

• use the regular expression we built in the previous step
• replace the whole string with the contents of the first capturing group: \\1 (we need to escape the backslash, hence the double backslash)

Example:

gsub(".*[_]([^.]+)[.].*", "\\1", "MA0051_IRF2.xml")

Answer 2

an other possibility with the stringr package:

 str_extract(x, perl("(?<=_)(.+)(?=\\.)"))