### LOCATION OF STRING

strings <- c("abcd", "cdab", "cabd", "c abd")

# anywhere in the text

grep("ab", strings, value = TRUE)


# at the beginning of a STRING

grep("^ab", strings, value = TRUE)

# at the end of a STRING

grep("ab$", strings, value = TRUE)

# at the beginning of a WORD

grep("\\bab", strings, value = TRUE)
## [1] "abcd"  "c abd"

# in the middle of a WORD

grep("\\Bab", strings, value = TRUE)



# replace land with LAND in all country names

gsub( "land", "LAND", c("finland", "iceland", "michael landon") )

# need to anchor the word to the end

gsub( "land$", "LAND", c("finland", "iceland", "michael landon") )




### QUANTIFIERS

strings <- c("ht","hot","hoot","hooot")

# match at least zero times

grep("h*t", strings, value = TRUE)

# match ONLY one time

grep("h.t", strings, value = TRUE)

# match at least one times

grep("ho+t", strings, value = TRUE)

# match zero or one times

grep("ho?t", strings, value = TRUE)

# match exactly n times

grep("ho{2}t", strings, value = TRUE)

# match at least n times

grep("ho{2,}t", strings, value = TRUE)

# match between n and m times

grep("ho{1,2}t", strings, value = TRUE)





strings <- c("ded", "dead","dad","deed", "deaad", "deeed" )

# wildcard character
grep( "dead", strings, value=TRUE )
grep( "de.d", strings, value=TRUE )

# quantifiers go after character x
# x? = 0 or 1 times 
# x* = 0 or more times 
# x+ = 1 or more times 

# equivalent to quantifiers:
# x? = x{0,1) 
# x* = x{0,}
# x+ = x{1,}

# any repeated letters
# .+ = .{1,)

grep( "dead", strings, value=TRUE )

grep( "dea?d", strings, value=TRUE )
grep( "dea{0,1}d", strings, value=TRUE )

grep( "dea*d", strings, value=TRUE )
grep( "dea{0,}d", strings, value=TRUE )

grep( "dea+d", strings, value=TRUE )
grep( "dea{1,}d", strings, value=TRUE )

grep( "de.+d", strings, value=TRUE )

# note operator position and scope: 
# ^d  e|a  d$
grep( "^de|ad$", strings, value=TRUE )




### USING THE OR OPERATOR

# FormA OR FormB OR FormC

my.text <- c( "FormA", "FormC", "FormE" )

grep( pattern="Form[ABC]", my.text )

grep( pattern="h[oi]t" , c("hot","hat","hit","hop") )


# note different OR options
# string1 | string2 (order matters)
# [abcd]  (set of letters)
#  e|a      e or a only
# [ea]{2}   ee, aa, ea, or ae
# [ea]{1,2} e, a, ee, aa, ea, or ae

strings <- c("ded", "dead","dad","deed", "deaad", "deeed" )

grep( "^de|ad$", strings, value=TRUE )
grep( "^dee|ad$", strings, value=TRUE )
grep( "^d[ea]d$", strings, value=TRUE )
grep( "^d[ea]{2}d$", strings, value=TRUE )
 


### ESCAPE OPERATOR

# Searching for special characters using escape

regexpr( "*", "abcd*efghi" )

regexpr( "\\*", "abcd*efghi" )






### EXAMPLES

strings <- c("^ab", "ab", "abc", "abd", "abe", "ab 12", "ab$")


# match anything that starts with ab followed by any character

grep("ab.", strings, value = TRUE)


# search for abc OR abd

grep("abc|abd", strings, value = TRUE)


# match abc OR abd OR abe

grep("ab[c-e]", strings, value = TRUE)


# match anything that is NOT abc

grep("ab[^c]", strings, value = TRUE)


# match any string where ab occurs at the beginning

grep("^ab", strings, value = TRUE)


# match any string where ab occurs at the end

grep("ab$", strings, value = TRUE)


# search for matches that contain the character ^

grep("^", strings, value = TRUE)


# try again

grep("\\^", strings, value = TRUE)


# same here

grep("$", strings, value = TRUE)

grep("\\$", strings, value = TRUE)





### PRACTICE - what will these return?

my.text <- c("micky","minnie","goofy","pluto")


grep( pattern="g*fy", my.text )


grep( pattern="pluo?to", my.text )


grep( pattern="pluo?t", my.text )


grep( pattern="plo?to", my.text )


grep( pattern="mi*", my.text )






# URL SLUG - no caps, spaces, or special chars
#
# <title> The Hangover Part 3 </title>
# <content> A silly comedy movie </content>
# <slug> the-hangover-part-3 </slug>


# regex to search for slugs in URLs: 
# match patterns xxx-xx-xxxxx-xxx

x <- c( "https://www.imdb.com/title/the-hangover-part-3", 
        "the-hangover-part-3", 
        "https://www.imdb.com/title/the-hangover-part-3/cast" )

grep( "[a-z0-9-]+", x, value=TRUE )
grep( "/[a-z0-9-]+/", x, value=TRUE )
grep( "^[a-z0-9-]+$", x, value=TRUE )


x <- c( "The Hangover Part 3",
        "the-hangover-part-3",  # slugified
        "The Hangover Part #", 
        "The-Hangover-Part-3",  # contains capitals
        "the-hangover-part-&" ) # contains special chars

# regex pseudocode: [a-z0-9-]
# contains lowercase a-z
# contains numbers 0-9
# contains dash -

grep( "^[a-z0-9-]+$", x, value=TRUE )
grep( "^[a-z0-9-]+$", x, value=TRUE, ignore.case=TRUE )
grep( "^[a-zA-Z0-9-]+$", x, value=TRUE, ignore.case=TRUE )