Regular Expressions (Regex)

A regular expression or Regex is a pattern or filter that describes a set of string that matches the pattern.

Basic Regex

Regex expression	Description	Ref
\	Escape character
+, *, ?	Quantifiers	[[#quantifiers----
.	Match words that contains character	[[#wildcard-
^	Start with a pattern, ^pattern
$	End with a pattern, pattern$
[ ]	Match either in the brackets, [xy]either x or y will be matched

Quantifiers (+ ,*, ?)

Quantifiers allow you to specify how many times a character or group of characters should be matched.

• +: Matches one or more occurrences.
• *: Matches zero or more occurrences.
• ?: Matches zero or one occurrence.

The ? find the lines that containing zero or one u.

grep "colou?r" colors.txt

The this case the program will find all lines containing either “color” or “colour”. The character before the ? sign is an option which is the u.

Wildcard (.)

The dot . matches any single character. For example, to find all three-letter words where the second letter is any character:

grep ".a." words.txt

You can use multiple dots to match multiple characters at specific positions. For example, to find all five-letter words where the first and third letters are “a” and the fourth letter is any character:

grep "a.a." words.txt

This will match words like “alas”.

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Bash Bash_scripting INFO1112 Regular_expression Regex