From regexone.com and regexr.com.
| Expression | Pattern | 
|---|---|
| abc... | Letters | 
| 123... | Digits | 
| [abc] | Any character in the set | 
| [^abc] | Any character not in the set | 
| [a-z] | Characters having a code between the two specified characters (inclusive) | 
| . | Any character | 
| \w | Any alphanumeric (A-Z, a-z, 0-9) character | 
| \W | Any non-alphanumeric character | 
| \d | Any digit | 
| \D | Any non-digit characters | 
| \s | Any whitespace (spaces, tabs, line breaks) | 
| \S | Any non-whitespace character | 
| Expression | Pattern | 
|---|---|
| ^ | Start of the string | 
| $ | End of the string | 
| \b | Boundary between a word character and non-word character | 
| \B | Any position that is not a word boundary | 
| Expression | Pattern | 
|---|---|
| * | Zero or more repetitions | 
| + | One or more repetitions | 
| ? | Optional character | 
| {m} | m repetitions | 
| {m,} | m or more repetitions | 
| {m, n} | m to n repetitions | 
| (abc|def) | abc or def | 
| Expression | Pattern | 
|---|---|
| \+ | Character with special meaning | 
| \xFF | Character in hexadecimal form | 
| \t | Tab character | 
| \n | Line feed character | 
| \r | Carriage return character | 
| \0 | Null character | 
| Expression | Pattern | 
|---|---|
| (abc) | Capture group | 
| \1 | Result of a capture group |