Regular Expressions Cheat Sheet - 2021

Regular expressions (regexes or regex patterns) are made available through the re module.

1. [abc]
   Matches any of the characters a, b, or c; this is the same as [a-c], which uses a range to express the same set of characters.
   If we want to match only lowercase letters, our RE should be [a-z].


2. [^5]
   Matches any character except '5'


3. [^0-9]
   Matches any non-digit character. This is equivalent to \D.


4. backslash, \
   
   1. \d
      matches any decimal digit; this is equivalent to [0-9].
   2. \D
      matches any non-digit character; this is equivalent to [^0-9]
   3. \s
      matches any whitespace character; this is equivalent to [ \t\n\r\f\v].
      [\s,.] will match any whitespace character, or ',' or '.'.
   4. \S
      matches any non-whitespace character; this is equivalent to [^ \t\n\r\f\v]
   5. \w
      matches any alphanumeric character; this is equivalent to [a-zA-Z0-9_]
   6. \W
      matches any non-alphanumeric character; this is equivalent to [^a-zA-Z0-9_]

1. *
   

   The '*' metacharacter doesn’t match the literal character '*'. Rather, it specifies that the previous character can be matched zero or more times, instead of exactly once.

   1. ca*t
      matches 'ct' (0 'a' characters), 'cat' (1 'a'), 'caaat' (3 'a' characters), and so forth.
   2. a[bcd]*b
      matches the letter 'a', zero or more letters from the class [bcd], and finally ends with a 'b'.
2. +
   matches one or more times. Note the difference between * and +
   * matches zero or more times, so whatever’s being repeated may not be present at all, while + requires at least one occurrence.
   
   1. ca+t
      matches 'cat' (1 'a'), 'caaat' (3 'a's), but won't match 'ct'.
3. ?
   matches either once or zero times
   
   1. home-?brew
      matches either 'homebrew' or 'home-brew'.
4. {m,n}
   there must be at least m repetitions, and at most n.
   
   1. a/{1,3}b
      matches 'a/b', 'a//b', and 'a///b'. It won’t match 'ab', which has no slashes, or 'a////b', which has four.