# General syntax:   
# grep(<pattern>, <character vector>)

mystring <- c('The number 5',
              'The number 8',
              '1 is the loneliest number',
              'Company, 3 is',
              'Git SSH tag is [email protected]',
              'My personal site is www.personal.org',
              'path/to/my/file')

grep('5', mystring)
# [1] 1
grep('@', mystring)
# [1] 5
grep('number', mystring)
# [1] 1 2 3

x|y means look for "x" or "y"

grep('5|8', mystring)
# [1] 1 2
grep('com|org', mystring)
# [1] 5 6

. is a special character in Regex. It means "match any character"

grep('The number .', mystring)
# [1] 1 2

Be careful when trying to match dots!

tricky <- c('www.personal.org', 'My friend is a cyborg')
grep('.org', tricky)
# [1] 1 2

To match a literal character, you have to escape the string with a backslash (\). However, R tries to look for escape characters when creating strings, so you actually need to escape the backslash itself (i.e. you need to double escape regular expression characters.)

grep('\.org', tricky)
# Error: '\.' is an unrecognized escape in character string starting "'\."
grep('\\.org', tricky)
# [1] 1

If you want to match one of several characters, you can wrap those characters in brackets ([])

grep('[13]', mystring)
# [1] 3 4
grep('[@/]', mystring)
# [1] 5 7

It may be useful to indicate character sequences. E.g. [0-4] will match 0, 1, 2, 3, or 4, [A-Z] will match any uppercase letter, [A-z] will match any uppercase or lowercase letter, and [A-z0-9] will match any letter or number (i.e. all alphanumeric characters)

grep('[0-4]', mystring)
# [1] 3 4
grep('[A-Z]', mystring)
# [1] 1 2 4 5 6

R also has several shortcut classes that can be used in brackets. For instance, [:lower:] is short for a-z, [:upper:] is short for A-Z, [:alpha:] is A-z, [:digit:] is 0-9, and [:alnum:] is A-z0-9. Note that these whole expressions must be used inside brackets; for instance, to match a single digit, you can use [[:digit:]] (note the double brackets). As another example, [@[:digit:]/] will match the characters @, / or 0-9.

grep('[[:digit:]]', mystring)
# [1] 1 2 3 4
grep('[@[:digit:]/]', mystring)
# [1] 1 2 3 4 5 7

Brackets can also be used to negate a match with a carat (^). For instance, [^5] will match any character other than "5".

grep('The number [^5]', mystring)
# [1] 2