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

Using rex to construct the regular expression may make it more understandable.

x <- c("_A00_A1234B_", "_A00_A12345B_", "_A1_A12345_")

# approach #1, assumes always is between the second underscores.
re_matches(x,
  rex(
    "_",
    anything,
    "_",
    capture(anything),
    "_"
  )
)

#>         1
#> 1  A1234B
#> 2 A12345B
#> 3  A12345


# approach #2, assumes an alpha, followed by 4 or 5 digits with a possible trailing alpha.
re_matches(x,
  rex(
    capture(
      alpha,
      between(digit, 4, 5),
      maybe(alpha)
    )
  )
)

#>         1
#> 1  A1234B
#> 2 A12345B
#> 3  A12345