Regular Expressions

The special characters (meta-characters) used for defining regular expressions are:

* . ^ $ + ? ( ) [ ] { } | \

Character sets and character classes

Operators	Description
`[abc]`	Match any single character from from the listed characters
`[a-z]`	Match any single character from the range of characters
`[^abc]`	Match any single character not among listed characters
`[^a-z]`	Match any single character not among listed range of characters
`.`	Match any single character except a newline (`\n`)
`\`	Turn off (escape) the special meaning of a metacharacter

Location-specific matches

Operators	Description
`^`	Match the beginning of a line.
`$`	Match the end of a line.

Bracket Expressions

Quantifier	Character Classes
`[:alnum:]`	Alphanumeric characters.
`[:alpha:]`	Alphabetic characters.
`[:blank:]`	Space and tab.
`[:digit:]`	Digits.
`[:lower:]`	Lowercase letters.
`[:upper:]`	Uppercase letters.

Repetitions, Grouping, and References

Operators	Description
`*`	Match zero or more instances of the preceding character or regex.
`?`	Match zero or one instance of the preceding character or regex.
`+`	Match one or more instances of the preceding character or regex.
`{n,m}`	Match a range of occurrences (at least n, no more than m) of preceding character of regex.
`\|`	Match the character or expression to the left or right of the vertical bar.

Here are some examples of repetitions:

[[:digit:]]* : any number of digits (zero or more)
[[:digit:]]+ : at least one digit
[[:digit:]]? : zero or one digits
[[:digit:]]{1,3} : at least one and no more than three digits
[[:digit:]]{2,} : two or more digits

Special Backslash Expressions

Expression	Description
`\b`	Match the empty string at the edge of a word.
`\B`	Match the empty string provided it’s not at the edge of a word.
`\<`	Match the empty string at the beginning of a word.
`\>`	Match the empty string at the end of a word.
`\w`	Match word constituent, it is a synonym for `[_[:alnum:]]`.
`\W`	Match non-word constituent, it is a synonym for `[^_[:alnum:]]`.
`\s`	Match whitespace, it is a synonym for `[[:space:]]`.
`\S`	Match non-whitespace, it is a synonym for `[^[:space:]]`.
`\d`	Match digit, it is a synonym for `[[:dight:]]`.
`\D`	Match non-digit, it is a synonym for `[^[:dight:]]`.

Greedy matching

Regular expression pattern matching is greedy—by default, the longest matching string is chosen.

.*

To get a non-greedy match, you can use the modifier ? after the quantifier. However, this requires that we use the Perl syntax. In order for grep to use the Perl syntax, we need to use the -P option.

.*?