Basic Matching

Any sequence of letters or digits will match exactly that sequence. For example, the regex “the” will match precisely those places where there is a “t” followed by an “h” followed by an “e”.

Meta Characters

• . : matches any single character except line break
• [] : character class, matches any character within the brackets
• [^] : negated character class, matches any character not in the square brackets
• * : matches 0 or more repetitions of the preceding symbol
• + : matches 1 or more repetitions of the preceding symbol
• ? : treats preceding symbol as optional match
• {n,m} : matches at least n but no more than m repetitions of preceding symbol
• () : capturing group, group of sub-patterns in parentheses. Makes it possible to extract pieces from the matching
• | : alternation, matches either characters before or characters after the “|”
• \ : escapes following character
• ^ : match at beginning of the input
• $ : match at the end of the input

Character Sets

• \w : matches alphanumeric characters [a-zA-z0-9_]
• \W : matches non-alphanumeric characters [^a-zA-z0-9_]
• \d : matches any digit [0-9]
• \D : matches any non-digit character [^\d]
• \s : matches whitespace characters
• \S : matches non-whitespace characters

Lookaround

• A(?=B): positive look ahead, match A when B is ahead/after
• A(?!B): negative look ahead, match A when B is not ahead/after
• (?<=B)A: positive look behind, match A when B is behind/before
• (?<!B)A: negative look ahead, match A when B is not behind/before