# regEx
## Anchors
| ^ | 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 |
## Character Classes
| \c | Control character |
| -- | ------------------ |
| \s | White space |
| \S | Not white space |
| \d | Digit |
| \D | Not digit |
| \w | Word |
| \W | Not word |
| \x | Hexadecimal digit |
| \O | Octal digit |
## POSIX
| \[:upper:] | Upper case letters |
| ----------- | ------------------------------ |
| \[:lower:] | Lower case letters |
| \[:alpha:] | All letters |
| \[:alnum:] | Digits and letters |
| \[:digit:] | Digits |
| \[:xdigit:] | Hexadecimal digits |
| \[:punct:] | Punctuation |
| \[:blank:] | Space and tab |
| \[:space:] | Blank characters |
| \[:cntrl:] | Control characters |
| \[:graph:] | Printed characters |
| \[:print:] | Printed characters and spaces |
| \[:word:] | Digits, letters and underscore |
## Assertions
| ?= | Lookahead assertion |
| ----------- | ------------------------- |
| ?! | Negative lookahead |
| ?<= | Lookbehind assertion |
| ?!= or ?\ | Once-only Subexpression |
| ?() | Condition \[if then] |
| ?()\| | Condition \[if then else] |
| ?# | Comment |
## Quantifiers
| \* | 0 or more | {3} | Exactly 3 |
| -- | --------- | ----- | --------- |
| + | 1 or more | {3,} | 3 or more |
| ? | 0 or 1 | {3,5} | 3, 4 or 5 |
Add a ? to a quantifier to make it ungreedy.
## Escape Sequences
| \\ | Escape following character |
| -- | -------------------------- |
| \Q | Begin literal sequence |
| \E | End literal sequence |
"Escaping" is a way of treating characters which have a special meaning in regular expressions literally, rather than as special characters.
## Common Metacharacters
| ^ | \[ | . | $ |
| - | -- | -- | -- |
| { | \* | ( | \\ |
| + | ) | \| | ? |
| < | > | | |
The escape character is usually \\
## Special Characters
| \n | New line |
| ---- | ------------------- |
| \r | Carriage return |
| \t | Tab |
| \v | Vertical tab |
| \f | Form feed |
| \xxx | Octal character xxx |
| \xhh | Hex character hh |
## Groups and Ranges
| . | 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.
## Pattern Modifiers
| 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
## String Replacement
| $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 $.