Regular expression examples in R

java_compatable_regex.txt

(?<!@|#)\b\w+    : Remove all words starting with @ or #  (remove hashtags and user handles from twitter)

(?<!@|#)\b\w{2,}  : Same as above but only keep words with length of 2 or greater

regex_examples.R

# Output of different grep commands in R ####

# Using a single string
# Is a number present?
example <- c("ABCD5EFG6")
# check if number is present
grep("[0-9]", example)
grepl("[0-9]", example)
# What location is the first number at
regexpr("[0-9]", example)
# Find the location of all digits
gregexpr("[0-9]", example)


# vector of strings
# Is a number present
example <- c("ABCD5EFG6","A7")
# check if number is present
grep("[0-9]", example)
grepl("[0-9]", example)
# What location is the first number at
regexpr("[0-9]", example)
# Find the location of all digits
gregexpr("[0-9]", example)


# Letters ####

# look for lower case letters
gregexpr("[a-z]", example)
# look for letters regardless of case
gregexpr("[a-z]", example, ignore.case = T)
# look for upper case letters
gregexpr("[A-Z]", example)

# Words ####
# find a word
example <- "two words"
gregexpr("\\w", example)
gregexpr("[0-9A-Za-z_]", example)

# find non-word character
gregexpr("\\W", example)
gregexpr("[^0-9A-Za-z_]", example)

# Spaces ####
# find spaces
gregexpr("\\s", example)
# find non-spaces
gregexpr("\\S", example)

# find digits ####
example <- "I'm looking for I95"
# find digits
gregexpr("\\d", example)
# find non-digits
gregexpr("\\D", example)

example <- "I95 or I676"
# find two consecutive digits
gregexpr("\\d\\d", example)
gregexpr("\\d{2}", example)

# find 3 consecutive digits
gregexpr("\\d\\d\\d", example)
gregexpr("\\d{3}", example)


# full stop "." ####
# find a then any character then e
example <- "What are your ages?"
gregexpr("a.e", example)

example <- "matter"
# no match found
gregexpr("a.e", example)


# Multipliers ####
example <- "cccccccc aaaaa"
gregexpr("c{3}", example)
gregexpr("c{2}", example)


# range multipliers
example <- "Chris Cris Chhris, Chhhhhris"
gregexpr("Ch{0,1}ris", example)
# ? is the same as using {0,1}
gregexpr("Ch?ris",example)

# 0 or more occurrences of h
gregexpr("Ch{0,}ris", example)
gregexpr("Ch*ris", example)