Regex

See also:
- man 7 regex
- Regex 101
- Regex tester
- RexEgg
- Regex for IP addresses

Match most non-English characters:

[^()[\]*+$?¿\/\-. \\~!@#$%^&*=_{}|;:․‥…·¨`‛''"‟’´‘’“”′‵″‶‴‷⁗„,<>—‒―¯–℃℉°№¹²³µ£¥¢©®℗™§¶ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789]

Match all duckduckgo subdomains except safe and help:
```
(?<!(safe|help))\.duckduckgo\.com
```

. = Any character.
[abc…] = Anything within brackets (a, b, or c).
[0-9] = Range within brackets (digits 0 through 9, inclusive).
[^123…] = Anything NOT within brackets.
\ = Escape next character.
( ) = Pattern grouping (([0-9]{1,3}\.){5} = 5 instances of 1-3 of any digit, followed by a period.).
- (?: ) = Non-capturing group. Use this when backreferences to a group aren't needed.
\1 = Backreference to group #1.
(?!foo) = Negative lookahead, matches any subsequent string that is NOT foo (similar to [^ ], except for a string rather than a range).
- grep -P '/documents/github-repos/(?!my-repos)' = Matches all directories under github-repos EXCEPT my-repos.

^ = Match pattern at start.
$ = Match pattern at end.
a|b = Alternation of patterns (a or b).
* = Zero or more of pattern.
- *? = Zero or more of pattern (lazy).
+ = One or more of pattern.
? = Zero or one of pattern.
Bounds: basic regex uses \{ \}, extended regex uses { }
- {1,5} = One to five of pattern.
- {3,} = At least three of pattern.
- {,2} = At most three of pattern.
- {1} = Exactly one of pattern.

Character classes	Similar to	GNU synonym	Only valid within brackets e.g [[:xyz:]]
`[:upper:]`	`[A-Z]`		Uppercase alphabetic characters
`[:lower:]`	`[a-z]`		Lowercase alphabetic characters
`[:digit:]`	`[0-9]`		Numeric characters
`[:alpha:]`	`[A-Za-z]`		Alphabetic characters
`[:alnum:]`	`[A-Za-z0-9]`	`\w`	Alphanumeric characters
`[:blank:]`	`[ \t]`		Space and tab characters ONLY
`[:space:]`	`[ \t\n\r\f\v]`	`\s`	Whitespace characters (space, tab, formfeed, etc.)
`[:cntrl:]`			Control characters
`[:graph:]`	`[^ [:cntrl:]]`		Graphical characters (non-control characters)
`[:print:]`	`[[:graph:] ]`		Graphical characters and space
`[:punct:]`			Punctuation characters (non-letter, digit, control char, or space)

GNU-style character classes
`\s`	Whitespace characters
`\S`	Non-whitespace characters
`\w`	letters, digits, underscores
`\W`	Non-letters, digits, underscores
`\f`	Form-feed
`\r`	Carriage return
`\n`	Newline
`\t`	Tab

Globbing