Definition
Regular expressions are patterns used to match character combinations. Look at given below table:
| Regular Expressions | Description |
| foo.* | # Matches any string starting with foo |
| \d* | # Match any number decimal digits |
| [a-zA-Z]+ | # Match a sequence of one or more letters |
| text | Match literal text |
| . | Match any character except newline |
| ^ | Match the start of a string |
| $ | Match the end of a string |
| * | Match 0 or more repetitions |
| + | Match 1 or more repetitions |
| ? | Match 0 or 1 repetitions *? Match 0 or more, few as possible |
| +? | Match 1 or more, few as possible |
| {m,n} | Match m to n repetitions |
| {m,n}? | Match m to n repetitions, few as possible |
| [...] | Match a set of characters |
| [^...] | Match characters not in set |
| A | B | Match A or B (...) Match regex in parenthesis as a group |
| \number | Matches text matched by previous group |
| \A | Matches start of string |
| \b | Matches empty string at beginning or end of word |
| \B | Matches empty string not at begin or end of word |
| \d | Matches any decimal digit |
| \D | Matches any non-digit |
| \s | Matches any whitespace |
| \S | Matches any non-whitespace |
| \w | Matches any alphanumeric character |
| \W | Matches characters not in |
| \w \Z | Match at end of string. |
| \\ | Literal backslash |