| \d | one digit from 0 to 9 |
| \w | word character: ASCII letter, digit or underscore |
| \s | whitespace character: space, tab, newline, carriage return, vertical tab |
| \D | One character that is not a digit |
| \W | One character that is not a word character ) |
| \S | One character that is not a whitespace character |
| \metachar | Escape a metacharacter “^{+<[*)>.(|$\?|” |
| \t | Tab |
| \r | Carriage return character |
| \n | Line feed character |
| \r\n | Line separator |
| a? | Zero or one of a |
| a* | Zero or more of a |
| a+ | One or more of a |
| [0-9]+ | One or more of 0-9 |
| a{3} | Exactly 3 of a |
| a{3,} | 3 or more of a |
| a{3,6} | Between 3 and 6 of a |
| a* | Greedy quantifier |
| a*? | Lazy quantifier |
| a*+ | Possessive quantifier |
| [abc] | A single character of: a, b or c |
| [^abc] | A character except: a, b or c |
| [a-z] | A character in the range: a-z |
| [^a-z] | A character not in the range: a-z |
| [0-9] | A digit in the range: 0-9 |
| [a-zA-Z] | A character in the range: a-z or A-Z |
| [a-zA-Z0-9] | A character in the range: a-z, A-Z or 0-9 |
| [\x41] | A character at hex position 41 in the ASCII table, i.e. A |
| ^ | Start of string |
| $ | End of string |
| \A | Start of string |
| \Z | End of string |
| \z | Absolute end of string |
| \b | A word boundary |
| \B | Non-word boundary |
| \G | Start of match |
| (…) | Capturing group: Match everything enclosed |
| (a|b) | Capturing group: Match either a or b |
| (?P<Y>…) | Capturing group named Y |
| (?:…) | Non-capturing group: Matches and does not remember the match |
| (?>…) | Atomic group (non-capturing) |
| (?|…) | Duplicate subpattern group number |
| (?=…) | Positive Lookahead. x(?=y)Matches “x” only if “x” is followed by “y” |
| (?!…) | Negative Lookahead. x(?!y) Matches “x” only if “x” is followed not by “y” |
| (?<=…) | Positive Lookbehind (?⇐y)x Matches “x” only if “x” is preceded by “y” |
| (?<!…) | Negative Lookbehind (?<!y)x Matches “x” only if “x” is not preceded by “y” |