How to align a set of strings on a character, with a given number of characters on each side (replace missing characters with "x")?

I have a set of strings, each has a single character "X"

c("KGDDQSXQGGAPDAGQE", "TEEDSEEVXEQK", "LTXTSGETTQTHTEPTGDSK", "IXTHNSEVEEDDMDK", "SXENPEEDEDQRNPAK", "XTAEHEAAQQDLQSK", "ATVIXHGETLRRTK", "XAVAREESGKPGAHVTVK", "YHTINGHNAEVXK", "XAAEDDEDDDVDTK")

I would like to get a character vector with each element having 11 characters, the center of the string is "X" and there is 5 characters from the string on each side. If there are no 5 characters on one of the sides, then "x" is added instead.

E.g.
"KGDDQSXQGGAPDAGQE", becomes "GDDQSXQGGAP"

"TEEDSEEVXEQK", becomes "DSEEVXEQKxx"

"LTXTSGETTQTHTEPTGDSK", becomes "xxxLTXTSGET"

Solution

One more approach, using stringr:

library(stringr)

vec <- c("KGDDQSXQGGAPDAGQE", "TEEDSEEVXEQK", "LTXTSGETTQTHTEPTGDSK", "IXTHNSEVEEDDMDK", "SXENPEEDEDQRNPAK", "XTAEHEAAQQDLQSK", "ATVIXHGETLRRTK", "XAVAREESGKPGAHVTVK", "YHTINGHNAEVXK", "XAAEDDEDDDVDTK")

vec %>%
  str_pad(width = sapply(vec, nchar) + 10, 
          side = "both", pad = "x") %>%
  str_match(".{5}X.{5}")
#>       [,1]         
#>  [1,] "GDDQSXQGGAP"
#>  [2,] "DSEEVXEQKxx"
#>  [3,] "xxxLTXTSGET"
#>  [4,] "xxxxIXTHNSE"
#>  [5,] "xxxxSXENPEE"
#>  [6,] "xxxxxXTAEHE"
#>  [7,] "xATVIXHGETL"
#>  [8,] "xxxxxXAVARE"
#>  [9,] "HNAEVXKxxxx"
#> [10,] "xxxxxXAAEDD"

^{Created on 2020-04-26 by the reprex package (v0.3.0)}