Literal characters
| Sr.No. | Example & Description |
|---|---|
| 1 | python Matches beginning of line. |
Character classes
| Sr.No. | Example & Description |
|---|---|
| 1 | [Pp]ython Match "Python" or "python" |
| 2 | rub[ye] Match "ruby" or "rube" |
| 3 | [aeiou] Match any one lowercase vowel |
| 4 | [0-9] Match any digit; same as [0123456789] |
| 5 | [a-z] Match any lowercase ASCII letter |
| 6 | [A-Z] Match any uppercase ASCII letter |
| 7 | [a-zA-Z0-9] Match any of the above |
| 8 | [^aeiou] Match anything other than a lowercase vowel |
| 9 | [^0-9] Match anything other than a digit |
Special Character Classes
| Sr.No. | Example & Description |
|---|---|
| 1 | . Match any character except newline |
| 2 | \d Match a digit: [0-9] |
| 3 | \D Match a nondigit: [^0-9] |
| 4 | \s Match a whitespace character: [ \t\r\n\f] |
| 5 | \S Match nonwhitespace: [^ \t\r\n\f] |
| 6 | \w Match a single word character: [A-Za-z0-9_] |
| 7 | \W Match a nonword character: [^A-Za-z0-9_] |
Repetition Cases
| Sr.No. | Example & Description |
|---|---|
| 1 | ruby? Match "rub" or "ruby": the y is optional |
| 2 | ruby* Match "rub" plus 0 or more ys |
| 3 | ruby+ Match "rub" plus 1 or more ys |
| 4 | \d{3} Match exactly 3 digits |
| 5 | \d{3,} Match 3 or more digits |
| 6 | \d{3,5} Match 3, 4, or 5 digits |
Nongreedy repetition
This matches the smallest number of repetitions −
| Sr.No. | Example & Description |
|---|---|
| 1 | <.*> Greedy repetition: matches "<python>perl>" |
| 2 | <.*?> Nongreedy: matches "<python>" in "<python>perl>" |
Grouping with Parentheses
| Sr.No. | Example & Description |
|---|---|
| 1 | \D\d+ No group: + repeats \d |
| 2 | (\D\d)+ Grouped: + repeats \D\d pair |
| 3 | ([Pp]ython(, )?)+ Match "Python", "Python, python, python", etc. |
Backreferences
This matches a previously matched group again −
| Sr.No. | Example & Description |
|---|---|
| 1 | ([Pp])ython&\1ails Match python&pails or Python&Pails |
| 2 | (['"])[^\1]*\1 Single or double-quoted string. \1 matches whatever the 1st group matched. \2 matches whatever the 2nd group matched, etc. |
Alternatives
| Sr.No. | Example & Description |
|---|---|
| 1 | python|perl Match "python" or "perl" |
| 2 | rub(y|le)) Match "ruby" or "ruble" |
| 3 | Python(!+|\?) "Python" followed by one or more ! or one ? |
Anchors
This needs to specify match position.
| Sr.No. | Example & Description |
|---|---|
| 1 | ^Python Match "Python" at the start of a string or internal line |
| 2 | Python$ Match "Python" at the end of a string or line |
| 3 | \APython Match "Python" at the start of a string |
| 4 | Python\Z Match "Python" at the end of a string |
| 5 | \bPython\b Match "Python" at a word boundary |
| 6 | \brub\B \B is nonword boundary: match "rub" in "rube" and "ruby" but not alone |
| 7 | Python(?=!) Match "Python", if followed by an exclamation point. |
| 8 | Python(?!!) Match "Python", if not followed by an exclamation point. |