| . | Any single character except newline (\n) |
| [abcde] | Any single one of the characters enclosed in the square brackets. |
| [a-z] | a-z is equivalent to all of the 26 low-case characters. - indicates the range. |
| [^a-z] | Match any single letter except a-z.^is a negated character when used right after [. |
| \d | [0-9] |
| \w | [a-zA-Z0-9_] |
| \s | \r\t\n\f |
| \D | [^0-9] |
| \W | [^a-zA-Z0-9_] |
| \S | [^ \r\t\n\f] |
| a* | * indicates "0 or more" of the immediately previous character (a in this example), or character class. |
| a+ | + indicates "1 or more" of the immediately previous character (a in this example), or character class. |
| a? | ? indicates "0 or 1" of the immediately previous character (a in this example), or character class. |
| a{5,10} | {5,10} indicates "5 to 10" of the immediately previous character (a in this example), or character class. One or the boundaries can be omitted. |
| a(.)b(.)c\2d\1 | match an a, a non-newline character (call it #1), a b, another non-newline character (call it #2), a c, the character #2, a d, and the character #1. |
| abc|defg | match abc or defg |
| fred\b | \b indicates a word boundary |
| fred\B | \B indicates NOT a word boundary |
| ^fred | ^ indicates the beginning of a string. It needs to appear as the first character |
| fred$ | $ indicates the end of a string. It needs to appear as the last character |