regex
Synopsis
Regular expressions (also referred to as rational expressions) are sequences of characters that specify a search pattern in the text. Such patterns are often used in string-searching algorithms to perform “find” and “find and replace” operations on strings, or to validate inputs.
Anchors
| Operators | Description |
|---|
^ | Start of string, or start of line in multi-line pattern |
\A | Start of string |
$ | End of string, or end of line in multi-line pattern |
\Z | End of string |
\b | Word boundary |
\B | Not word boundary |
\< | Start of word |
\> | End of word |
Quantifiers
| Operators | Occurrence | Example | Description |
|---|
? | 0 or 1 | {3,5} | 3, 4 or 5 |
* | 0 or more | {3} | Exactly 3 |
+ | 1 or more | {3,} | 3 or more |
Add a ? to a quantifier to make it ungreedy.
Groups and Ranges
| Operator | Description |
|---|
. | Any character except new line (\n) |
(a|b) | a or b |
(...) | Group |
(?:...) | Passive (non-capturing) group |
[abc] | Range (a or b or c) |
[^abc] | Not (a or b or c) |
[a-q] | Lower case letter from a to q |
[A-Q] | Upper case letter from A to Q |
[0-7] | Digit from 0 to 7 |
\x | Group/subpattern number “x” |
Ranges are inclusive.
Escape Sequences
| Operator | Description |
|---|
\ | Escape following character |
\Q | Begin literal sequence |
\E | End literal sequence |
Escaping is a way of treating characters which have a special meaning in regex literally, rather than as special characters.
The escape character is usually \
Special Characters
| Operator | Description |
|---|
\n | New line |
\r | Carriage return |
\t | Tab |
\v | Vertical tab |
\f | Form feed |
\xxx | Octal character xxx |
\xhh | Hex character hh |
Assertions
| Operator | Description |
|---|
?= | Lookahead assertion |
?! | Negative lookahead |
?<= | Lookbehind assertion |
?!=
or
?<! | Negative lookbehind |
?> | Once-only SubExpression |
?() | Condition [if then] |
?()| | Condition [if then else] |
?# | Comment |
String Replacement
| Operator | Description |
|---|
$n | nth non-passive group |
$2 | “xyz” in /^(abc(xyz))$ |
$1 | “xyz” in /^(?:abc)(xyz)$/ |
$` | Before matched string |
$' | After matched string |
$+ | Last matched string |
$& | Entire matched string |
Some regex implementations use \ instead of $.
Pattern Modifiers
| Operator | Description |
|---|
g | Global match |
i * | Case-insensitive |
m * | Multiple lines |
s * | Treat string as single line |
x * | Allow comments and whitespace in pattern |
e * | Evaluate replacement |
U * | Ungreedy pattern |
* —> PCRE modifier