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  1. library(stringr)
  2.  
  3. x <- c('ondaX', 'ondasX', 'ondasssX', 'onda', 'ondas')
  4.  
  5. str_extract(x, regex('ondas??X'))
  6.  
  7. str_extract(x, regex('ondas*?X'))
Success #stdin #stdout 0.29s 42476KB
stdin
Standard input is empty
stdout
[1] "ondaX"  "ondasX" NA       NA       NA      
[1] "ondaX"    "ondasX"   "ondasssX" NA         NA