str_extract specific patterns (example)

Question

I\'m still a little confused by regex syntax. Can you please help me with these patterns:

_A00_A1234B_
_A00_A12345B_
_A1_A12345_

my approac

Answer 1

You can do this without using a regular expression ...

x <- c('_A00_A1234B_', '_A00_A12345B_', '_A1_A12345_')
sapply(strsplit(x, '_', fixed=T), '[', 3)
# [1] "A1234B"  "A12345B" "A12345" 

If you insist on using a regular expression, the following will suffice.

regmatches(x, regexpr('[^_]+(?=_$)', x, perl=T))