5 Strings

5.1 Basic string operations

1. How to use regular expressions?

    stringr::str_detect("1", "\\d")
#> [1] TRUE
    # is the same as
    stringr::str_detect("1", stringr::regex("\\d"))
#> [1] TRUE
    #   ____________________________________________________________________________

1. Paste/concatenate strings.

   stringr::str_c("a", "b", NA, NaN, NULL, factor("lalala"), character(0))
   #> [1] NA
   #   ____________________________________________________________________________
   paste("a", "b", NA, NaN, NULL, factor("lalala"), character(0), sep = "")
   #> [1] "abNANaNlalala"

2. Treat NA’s as strings ("NA").

   stringr::str_replace_na(c(NA, NaN, factor("lalala"), character(0)))
   #> [1] "NA"  "NaN" "1"
   # note that the `c` function coerces the factor to its integer representation
   #   ____________________________________________________________________________
   base_replace_na <- function(x) {ifelse(is.na(x), "NA", x)}
   base_replace_na(c(NA, NaN, factor("lalala"), character(0)))
   #> [1] "NA"  "NaN" "1"

3. Count the number of letters in a string.

   stringr::str_length(c("four", NA, NaN, NULL, factor("lalala")))
   #> [1]  4 NA  3  1
   #   ____________________________________________________________________________
   nchar(c("four", NA, NaN, NULL, factor("lalala")))
   #> [1]  4 NA  3  1

4. Return the last letter of a string, subset a string.

   stringr::str_sub(c("five", "six"), -1)
   #> [1] "e" "x"
   #   ____________________________________________________________________________
   substr(c("five", "six"), 
          start = nchar(c("five", "six")), 
          stop = nchar(c("five", "six")))
   #> [1] "e" "x"

5. Repeat/replicate/duplicate a string several times.

   stringr::str_dup("_a_", 5)
   #> [1] "_a__a__a__a__a_"
   #   ____________________________________________________________________________
   paste(rep("_a_", 5), collapse = "")
   #> [1] "_a__a__a__a__a_"

6. Remove/trim spaces at the beginning (leading) and/or end (trailing) of a string.

   stringr::str_trim(c("  2left_1right ", "   "), side = c("left"))
   #> [1] "2left_1right " ""
   #   ____________________________________________________________________________
   # ^\\s+|\\s+$ is the regex for one or more spaces in the beginning
   # or one or more spaces at the end
   gsub("^\\s+", "", c("  2left_1right ", "   "))
   #> [1] "2left_1right " ""

7. Fill/pad spaces (or other characters) before and/or after a string.

   stringr::str_pad(c("lalala", ""), width = 7)
   #> [1] " lalala" "       "
   #   ____________________________________________________________________________
   unname(
     vapply(c("lalala", ""), 
            function(x) if(nchar(x) < 7) {
              paste0(rep(" ", 7 - nchar(x)), x, collapse = "")
              } else {x},
            character(1)
            )
     )
   #> [1] " lalala" "       "

8. Sort strings for example, by a different local alphabetic order.

   # The vowels come before the consonants in Hawaiian
   stringr::str_sort(letters[1:10], locale = "haw")
   #>  [1] "a" "e" "i" "b" "c" "d" "f" "g" "h" "j"
   #   ____________________________________________________________________________
   # one can use `sort()`, but one has to know the specific locale

9. Return the order of stringelements within a specific alphabet

   # The vowels come before the consonants in Hawaiian
   stringr::str_order(letters[1:10], locale = "haw")
   #>  [1]  1  5  9  2  3  4  6  7  8 10
   #   ____________________________________________________________________________
   # one can use `sort()`, but one has to know the specific locale

10. Change the encoding of a string

    # Example from encoding?stringi::stringi
    x <- rawToChar(as.raw(177))
    x
    #> [1] "±"
    stringr::str_conv(x, "ISO-8859-2") # Polish "a with ogonek", (run this line in RStudio)
    #> [1] "a"
    stringr::str_conv(x, "ISO-8859-1") # Plus-minus
    #> [1] "±"
    #   ____________________________________________________________________________

11. Return the first two words of a string

    stringr::word("Hello, how are you?", 1, 2)
    #> [1] "Hello, how"
    #   ____________________________________________________________________________

12. Format a string (change width, indentaion, …).

    cat(stringr::str_wrap("Hello, how are you?", width = 10, indent = 0, exdent = 2))
    #> Hello, how
    #>   are you?
    #   ____________________________________________________________________________

5.2 Pattern matching

1. How to disable case sensitivity in regex pattern?

   stringr::str_detect(c("a", "b", "c"), stringr::regex("b|A|D", ignore_case = TRUE))
   #> [1]  TRUE  TRUE FALSE

2. Does a string contain a specific sequence of characters?

   stringr::str_detect(c("my number is 110", "call 911"), "\\d{3}")
   #> [1] TRUE TRUE
   #   ____________________________________________________________________________
   grepl("\\d{3}", c("my number is 110", "call 911"))
   #> [1] TRUE TRUE

3. Return every string that contains a specific sequence of characters?

   stringr::str_subset(c("my number is 110", "call 911"), "\\d{3}")
   #> [1] "my number is 110" "call 911"
   #   ____________________________________________________________________________
   grep("\\d{3}", c("my number is 110", "call 911"), value = TRUE)
   #> [1] "my number is 110" "call 911"

4. Return the (first) start and end index of substrings within a string?

   # use stringr::str_locate_all to get a matrix of all start and end indexes
   stringr::str_locate(c("my 112 number is 110", "call 911"), c("\\d{3}"))
   #>      start end
   #> [1,]     4   6
   #> [2,]     6   8
   #   ____________________________________________________________________________
   # Use gregexpr to construct something similar to str_locate_all
   matches <- regexpr("\\d{3}", c("my 112 number is 110", "call 911"))
   
   # formatting
   match_lengths <- attr(matches, "match.length")
   attributes(matches) <- NULL
   match_indices <- matrix(c(matches, matches + match_lengths - 1), ncol = 2)
   colnames(match_indices) <- c("start", "end")
   
   # output
   match_indices
   #>      start end
   #> [1,]     4   6
   #> [2,]     6   8

5. Return the (first) matched substring within a string?

   # use stringr::str_extract_all to get a matrix of all matched substrings
   stringr::str_extract(c("my 112 number is 110", "call 911"), c("\\d{3}"))
   #> [1] "112" "911"
   #   ____________________________________________________________________________
   # combine `substr()` with match_indices from above

6. Return the matching strings and matched capture groups.

   # the example from the stringr vignette
   strings <- c(
     "apple",
     "219 733 8965", 
     "329-293-8753", 
     "Work: 579-499-7527; Home: 543.355.3679"
     )
   phone <- "([2-9][0-9]{2})[- .]([0-9]{3})[- .]([0-9]{4})"
   
   stringr::str_match(strings, phone)
   #>      [,1]           [,2]  [,3]  [,4]  
   #> [1,] NA             NA    NA    NA    
   #> [2,] "219 733 8965" "219" "733" "8965"
   #> [3,] "329-293-8753" "329" "293" "8753"
   #> [4,] "579-499-7527" "579" "499" "7527"
   # Use stringr::str_match_all for list return
   #   ____________________________________________________________________________
   # `regmatches()`

7. Replace/substitute the first matching subtstring within a string.

   stringr::str_replace(c("my 112 number is 110", "call 911"),
                        c("\\d{1}(\\d{2})"), "\\1\\1")
   #> [1] "my 1212 number is 110" "call 1111"
   #   ____________________________________________________________________________
   sub(c("\\d{1}(\\d{2})"), "\\1\\1", c("my 112 number is 110", "call 911"))
   #> [1] "my 1212 number is 110" "call 1111"

8. Replace/substitute all matching subtstrings within a string.

   stringr::str_replace_all(c("my 112 number is 110", "call 911"),
                            c("\\d{1}(\\d{2})"), "\\1\\1")
   #> [1] "my 1212 number is 1010" "call 1111"
   #   ____________________________________________________________________________
   gsub(c("\\d{1}(\\d{2})"), "\\1\\1", c("my 112 number is 110", "call 911"))
   #> [1] "my 1212 number is 1010" "call 1111"

9. Split strings by any pattern into different (optionally n) pieces.

   # returns a matrix and padds with empty strings, when n is greater than the
   # number of pieces
   stringr::str_split_fixed(c("lalala lala la",
                              "blabla lala la"),
                            c(" ", "b"), 5)
   #>      [,1]     [,2]   [,3]         [,4] [,5]
   #> [1,] "lalala" "lala" "la"         ""   ""  
   #> [2,] ""       "la"   "la lala la" ""   ""
   # returns a list
   stringr::str_split(c("lalala lala la",
                        "blabla lala la"),
                      c(" ", "b"), 5)
   #> [[1]]
   #> [1] "lalala" "lala"   "la"    
   #> 
   #> [[2]]
   #> [1] ""           "la"         "la lala la"
   #   ____________________________________________________________________________
   # returns a list, but does not have an n argument (however, it shouldn't be
   # too hard to create one).
   strsplit(c("lalala lala la",
              "blabla lala la"),
            c(" ", "b"))
   #> [[1]]
   #> [1] "lalala" "lala"   "la"    
   #> 
   #> [[2]]
   #> [1] ""           "la"         "la lala la"

10. Set all characters to lower/upper case.

    stringr::str_to_lower("aBcDe")
    #> [1] "abcde"
    stringr::str_to_upper("aBcDe")
    #> [1] "ABCDE"
    #   ____________________________________________________________________________
    tolower("aBcDe")
    #> [1] "abcde"
    toupper("aBcDe")
    #> [1] "ABCDE"

11. Truncate a string.

    stringr::str_trunc("123456789", width = 7, side = "right", ellipsis = "")
    #> [1] "1234567"
    #   ____________________________________________________________________________

5.2.1 Regular expressions

1. Where can I learn about regular expressions?

• ?stringr::modifiers for special regular expression functionality of the stringr package.
• reference sheet
• interactively test
• Build a regular expressin
• Try the str_view() function to see, which substring a regex matches

```r
stringr::str_view(c("abc", "a.c_blabla", "bef"), "a\\.c")
```


    

1. How to test regular expressions?

When writing regular expressions, I strongly recommend generating a list of positive (pattern should match) and negative (pattern shouldn’t match) test cases to ensure that you are matching the correct components.

5.2.2 Modifiers

1. Count the number of words within a string.

   words <- c("These are some words. 
              Some more words.")
   stringr::str_count(words, stringr::boundary("word"))
   #> [1] 7
   #   ____________________________________________________________________________

2. Count the number of lines within a string.

   stringr::str_count(words, stringr::boundary("line_break")) -
   stringr::str_count(words, stringr::boundary("word"))
   #> [1] 1
   #   ____________________________________________________________________________

3. Count the number of sentences within a string.

   stringr::str_count(words, stringr::boundary("sentence"))
   #> [1] 2
   #   ____________________________________________________________________________

4. Split a string by characters.

   stringr::str_split(words, stringr::boundary("character"))
   #> [[1]]
   #>  [1] "T"  "h"  "e"  "s"  "e"  " "  "a"  "r"  "e"  " "  "s"  "o"  "m"  "e" 
   #> [15] " "  "w"  "o"  "r"  "d"  "s"  "."  " "  "\n" " "  " "  " "  " "  " " 
   #> [29] " "  " "  " "  " "  " "  " "  "S"  "o"  "m"  "e"  " "  "m"  "o"  "r" 
   #> [43] "e"  " "  "w"  "o"  "r"  "d"  "s"  "."
   #   ____________________________________________________________________________
   strsplit(words, "")
   #> [[1]]
   #>  [1] "T"  "h"  "e"  "s"  "e"  " "  "a"  "r"  "e"  " "  "s"  "o"  "m"  "e" 
   #> [15] " "  "w"  "o"  "r"  "d"  "s"  "."  " "  "\n" " "  " "  " "  " "  " " 
   #> [29] " "  " "  " "  " "  " "  " "  "S"  "o"  "m"  "e"  " "  "m"  "o"  "r" 
   #> [43] "e"  " "  "w"  "o"  "r"  "d"  "s"  "."

5.3 List output

1. How to work with string functions, that return lists?

5.4 Resources

• stringr vignette
• [Regular Expressions in R - Cheatsheet]https://www.rstudio.com/wp-content/uploads/2016/09/RegExCheatsheet.pdf