^ = Beginning of Line or String
$ = End of Line or String
. = Matches Anything like a Joker Card (inc blank spaces)
. = Escape the period when we search on an actual period
d or [::digit::] = 0, 1, 2, 3, ...
w or [::word::] = a, b, c, ..., 1, 2, 3, ..., _
[A-Za-z] or [::alpha::] = A, B, C, ... a, b, c, ...
[aeiou] = a, e, i, o, u
s or [::space::] = " ", tabs or line breaks
D = match all except digits
W = match all except word char (inc numbers and underscore)
S = match all except spaces, tabs or line breaks
[^A-Za-z] = match all except alphabet
d{2} = repeat digit exactly twice (double digit)
d{2, 3} = min repeat digit twice, max repeat 3 thrice
d{2,} = min repeat digit twice, no max
d+ = 1 or more repetitions
d* = 0, 1 or more repetitions
| = OR (pattern = "apple|apples") [Not limited to 2 options only]
# No spaces are allowed within the vertical bar(s)
? = make the preceding group or character optional or
make the preceding multiplier "lazy" instead of "greedy".
Similar as above when coded as (pattern = apples?)
Greedy and Lazy [See Example 5a and b]
pattern = ".*3" (match everything before the last digit of 3)[Greedy]
pattern = ".*?3" (match only everything before the first digit of 3)[Lazy]
Examples:
1) Find all digits and spaces
stringr::str_match_all(x, pattern = "[ds]")
2) List all strings that end with a space followed by a digit
data[stringr::str_detect(x, pattern = "sd$")]
3) List all strings that contain "Grey" or "Gray"
data[stringr::str_detect(x, pattern = "Gr[ae]y")]
4) List all strings with strange characters (no word or space)
data[stringr::str_detect(x, pattern = "[^ws]")]
5) Greedy vs Lazy
a) Greedy
>stringr::str_match("a 3 b 3 c", ".*3")
"a 3 b 3"
b) Lazy
>stringr::str_match("a 3 b 3 c", ".*?3")
"a 3"
6) Three or more word characters, followed by an at-sign,
one or more word characters and a ."com".
pattern = "w{3,}@w+.com"
x <- "1888 is the longest year in Roman numerals: MDCCCLXXXVIII"
str_extract(x, "CC?")
#> [1] "CC"
str_extract(x, "CC+")
#> [1] "CCC"
str_extract(x, 'C[LX]+')
#> [1] "CLXXX"