If you work with text, you’ll appreciate how useful regular expressions are. Regular expressions are everywhere in Linux for searching through text right down to the character. This Regular Expressions cheatsheet will be useful for people who simply need a little refresher from time to time.
| Character | Description | Example |
|---|---|---|
| . (dot) | Match any single character, except newline (\n) | c.t matches "cat", "cut" or "cot." |
| * (star) | Repeat the previous expression 0 or more times (greedy mode) | 12*3 matches "13", "123", "1223", "12223". It can be use together with . (dot) such as m.*easier matches "maketecheasier". Using .* by itself is meaningless as it matches everything and return the full result. |
| + (plus) | Repeat the previous expression 1 or more times. | 12+3 matches "123","1223","12223" |
| ? (question mark) | Makes the previous item optional. | ma?ke matches "make", "mke" |
| ^ (caret) | Match from the beginning of the string | ^he matches "hello", "hell", "help", "he is a boy" |
| $ (dollar) | Match from the end of the string | ed$ matches "acted", bed", "greed" |
| (...) (round bracket) | Grouping of characters or expression | (ak) matches "make", "take", ' |
| {n} (curly bracket, where n is an integer bigger than 0) | Match the previous item exactly n times | 12{3}5 matches "12225" |
| [...] (square bracket) | match a single character in the bracket | [abc] matches "a","b" or "c" in the string "abc". |
| [^...] | Match any character except for those that are defined in the bracket | a[^b]c matches "aec", "acc", "adc", but not "abc" |
| | (pipe) | Match either the expression on the left or right of the pipe. | col(o|ou)r matches "color", "colour" |
| - (hypen) | Specify a range of characters to match. Used mostly in [a-z], [A-Z],[1-9],[a-zA-Z1-9] | a[a-z]c matches "abc", "acc", "adc" |
| \ (backslash) | Escape a special character and turn it into an ordinary character. | a\*c matches "a*c". |
| \n, \r, \t | match a newline, return and tab character respectively | |
| \b...\b | Match a word within the boundary. | \bTech\b matches the word "Tech" in "Make Tech Easier". |
Some More Complex Examples
Matching a certain number of characters.
Here’s an example for a U.S. phone number, not counting the area code:
[0-9]{3}-[0-9]{4}
This will match any phone number of the format “111-1111”.
Making a pattern optional
Here’s the U.S. phone number example again, this time with optional area codes. We’ll assume that the file with the phone numbers we’re looking for has phone numbers structured like this: 555-555-5555. The “?” operator denotes an optional pattern preceding it.
([0-9]{3}-)?[0-9]{3}-[0-9]{4}
Sure, it’s a bit ugly, as regular expressions tend to be, but it’s very powerful.
Finding a range of characters:
.{1,3}
This will match any character between 1 and 3 times.
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