Regular expressions are patterns used in UNIX commands to match the contents of a file.
Regular expression wildcards, also called metacharacters, are used to define the search parameters.
They include the following:
^ Beginning of Line (Must be first character in your pattern)
$ End of Line (Must be the last character in your pattern )
. Any one character
[aprw] Matches one character from the listed set. This notation is called a character class.
[a-c4-8] The listed set may include ranges of characters. Ranges are indicated using a dash between two characters.
This set includes all the characters from a to c, plus the digits from 4 to 8.
[^a-r] You can reverse the meaning of the set by inserting a caret as the first character.
This matches any one character except a through r.
Matching Any Character
You could also match those pesky hyphens with a dot (.):
The dot or period essentially acts as a wildcard and will match any character (except, in certain situations, a line ending). In the example above, the regular expression matches the hyphen, but it could also match a percent sign (%):
Or a vertical bar (|):
Or any other character.
the dot character (officially, the full stop) will not normally match a new line character, such as a line feed (U+000A).
However, there are ways to make it possible to match a newline with a dot, which I will show you later. This is often called the dotall option.