RegEx
Anchors
| Syntax | Description |
| ^ | Start of string, or start of line in multi-line pattern |
| $ | End of string, or end of line in multi-line pattern |
Quantifiers
| Syntax | Description |
| * | 0 or more |
| ? | 0 or 1 |
| + | 1 or more |
| {3} | Exactly 3 |
| {3,} | 3 or more |
| {3,5} | 3, 4 or 5 |
| note | Add a ? to a quantifier to make it ungreedy |
Escape
| Syntax | Description |
| \ | Escape following character |
Escaping is a way of treating characters which have a special meaning in regular expressions literally, rather than as special characters.
Groups and Ranges
| Syntax | Description |
| . | Any character except new line (\n) |
| (a|b) | a or b |
| (…) | Group, capturing |
| (?:…) | 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 |
Use a group to “capture and replace”.
a(bc) parentheses create a capturing group with value bc
The parentheses are used to create “groups,” which then get assigned a base-1 index, accessible in a replace with a $ (i.e., the first parentheses-bounded (characters) become a group that can be referenced by $1, the second group would be $2, the third $3, etc.).
NOTE: in gedit you reference with a backslash: \ not a $ (i.e., \1\2\3, etc.).
See the follow table regarding string replacements.
String Replacement
| Syntax | 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 |
Note: Some regex implementations use \ instead of $.
Character Classes
| Syntax | Description |
| \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
| Syntax | Description |
| [: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 |