How to check whether a string is in lowercase or uppercase in R?

We can use str_detect function to check whether a single string or a vector of strings is in lowercase or uppercase. Along with str_detect function, we need to use either upper or lower to check whether the string is in lowercase or uppercase and the output will be returned in TRUE or FALSE form, if the string will be in lowercase and we pass lower with str_detect function then the output will be TRUE and vice-versa.
 Live Demo
x1<-letters[1:26]
x1
[1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s"
[20] "t" "u" "v" "w" "x" "y" "z"
str_detect(x1,"[[:upper:]]")
[1]  FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[25] FALSE FALSE
 Live Demo
x2<-LETTERS[1:26]
x2
[1] "A" "B" "C" "D" "E" "F" "G" "H" "I" "J" "K" "L" "M" "N" "O" "P" "Q" "R" "S"
[20] "T" "U" "V" "W" "X" "Y" "Z"
str_detect(x2,"[[:upper:]]")
[1] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE
[16] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE
 Live Demo
x3<-"india"
x3
[1] "india"
str_detect(x3,"[[:upper:]]")
[1] FALSE

 Live Demo
x4<-c("abc","abcd","abcde","bacdef")
x4
[1] "abc" "abcd" "abcde" "bacdef"
str_detect(x4,"[[:upper:]]")
[1] FALSE FALSE FALSE FALSE

 Live Demo
x5<-c("AK", "AL", "AR", "AS", "AZ", "CA", "CO", "CT", "DC", "DE", "FL", "GA", "GU", "HI", "IA", "ID", "IL", "IN", "KS", "KY", "LA", "MA", "MD", "ME", "MI", "MN", "MO", "MP", "MS", "MT", "NC", "ND", "NE", "NH", "NJ", "NM", "NV", "NY", "OH", "OK", "OR", "PA", "PR", "RI", "SC", "SD", "TN", "TX", "UM", "UT", "VA", "VI", "VT", "WA", "WI", "WV", "WY")
x5
[1] "AK" "AL" "AR" "AS" "AZ" "CA" "CO" "CT" "DC" "DE" "FL" "GA" "GU" "HI" "IA"
[16] "ID" "IL" "IN" "KS" "KY" "LA" "MA" "MD" "ME" "MI" "MN" "MO" "MP" "MS" "MT"
[31] "NC" "ND" "NE" "NH" "NJ" "NM" "NV" "NY" "OH" "OK" "OR" "PA" "PR" "RI" "SC"
[46] "SD" "TN" "TX" "UM" "UT" "VA" "VI" "VT" "WA" "WI" "WV" "WY"
str_detect(x5,"[[:upper:]]")
[1] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE
[16] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE
[31] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE
[46] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE
 Live Demo
x6<-c("alabama", "alaska", "american samoa", "arizona", "arkansas", "california", "colorado", "connecticut", "delaware", "district of columbia", "florida", "georgia", "guam", "hawaii", "idaho", "illinois", "indiana", "iowa", "kansas", "kentucky", "louisiana", "maine", "maryland", "massachusetts", "michigan", "minnesota", "minor outlying islands", "mississippi", "missouri", "montana", "nebraska", "nevada", "new hampshire", "new jersey", "new mexico", "new york", "north carolina", "north dakota", "northern mariana islands", "ohio", "oklahoma", "oregon", "pennsylvania", "puerto rico", "rhode island", "south carolina", "south dakota", "tennessee", "texas", "u.s. virgin islands", "utah", "vermont", "virginia", "washington", "west virginia", "wisconsin", "wyoming")
x6
[1] "alabama" "alaska"
[3] "american samoa" "arizona"
[5] "arkansas" "california"
[7] "colorado" "connecticut"
[9] "delaware" "district of columbia"
[11] "florida" "georgia"
[13] "guam" "hawaii"
[15] "idaho" "illinois"
[17] "indiana" "iowa"
[19] "kansas" "kentucky"
[21] "louisiana" "maine"
[23] "maryland" "massachusetts"
[25] "michigan" "minnesota"
[27] "minor outlying islands" "mississippi"
[29] "missouri" "montana"
[31] "nebraska" "nevada"
[33] "new hampshire" "new jersey"
[35] "new mexico" "new york"
[37] "north carolina" "north dakota"
[39] "northern mariana islands" "ohio"
[41] "oklahoma" "oregon"
[43] "pennsylvania" "puerto rico"
[45] "rhode island" "south carolina"
[47] "south dakota" "tennessee"
[49] "texas" "u.s. virgin islands"
[51] "utah" "vermont"
[53] "virginia" "washington"
[55] "west virginia" "wisconsin"
[57] "wyoming"
str_detect(x6,"[[:upper:]]")
[1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[25] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[37] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
[49] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE