r - Change name of certain character and location in filenames

Question

I want to change one of the _ to another character, for example to -, the reason is there are problems reading in these filenames. I want a to become like b. So I want to change the second last underscore(_), how to specify this in an efficient way?

gsub("_", "-"), it must also be specified to a certain location.

a <- c("2018-01-09_B2_HILIC_POS_123_-14b_090.mzML", "2018-01-09_B2_HILIC_POS_243_-12a_026.mzML", "2020-01-09_B2_HILIC_POS_415_893a_059.mzML", "2020-01-18_B3_HILIC_POS_LV7001248356_040.mzML")
b <- c("2018-01-09_B2_HILIC_POS_123--14b_090.mzML", "2018-01-09_B2_HILIC_POS_243--12a_026.mzML", "2020-01-09_B2_HILIC_POS_415-893a_059.mzML", "2020-01-18_B3_HILIC_POS_LV4004365711_040.mzML")

question from:https://stackoverflow.com/questions/65880433/change-name-of-certain-character-and-location-in-filenames

Answer 1

Use

sub("_(?=[^_]*_[^_]*$)", "-", a, perl=TRUE)

See regex proof.

Explanation

--------------------------------------------------------------------------------
  _                        '_'
--------------------------------------------------------------------------------
  (?=                      look ahead to see if there is:
--------------------------------------------------------------------------------
    [^_]*                    any character except: '_' (0 or more
                             times (matching the most amount
                             possible))
--------------------------------------------------------------------------------
    _                        '_'
--------------------------------------------------------------------------------
    [^_]*                    any character except: '_' (0 or more
                             times (matching the most amount
                             possible))
--------------------------------------------------------------------------------
    $                        before an optional 
, and the end of
                             the string
--------------------------------------------------------------------------------
  )                        end of look-ahead

See R proof:

a <- c("2018-01-09_B2_HILIC_POS_123_-14b_090.mzML", "2018-01-09_B2_HILIC_POS_243_-12a_026.mzML", "2020-01-09_B2_HILIC_POS_415_893a_059.mzML", "2020-01-18_B3_HILIC_POS_LV7001248356_040.mzML")
sub("_(?=[^_]*_[^_]*$)", "-", a, perl=TRUE)

Results:

[1] "2018-01-09_B2_HILIC_POS_123--14b_090.mzML"    
[2] "2018-01-09_B2_HILIC_POS_243--12a_026.mzML"    
[3] "2020-01-09_B2_HILIC_POS_415-893a_059.mzML"    
[4] "2020-01-18_B3_HILIC_POS-LV7001248356_040.mzML"