The ? quantifier makes the (fear) group optional: $ grep -E '(fear)?less' file.txt Special Backslash Expressions The following example matches both “fearless” and “less”. When using basic regular expressions, the parenthesis must be escaped with a backslash ( \). Grouping is a feature of the regular expressions that allows you to group patterns together and reference them as one item. ![]() If you use the extended regular expression, then the operator | should not be escaped, as shown below: $ grep -E 'fatal|error|critical' /var/log/nginx/error.log Grouping In the example below, we are searching for all occurrences of the words fatal, error, and critical in the Nginx log error file: $ grep 'fatal\|error\|critical' /var/log/nginx/error.log This operator has the lowest precedence of all regular expression operators. The alternation operator | (pipe) allows you to specify different possible matches that can be literal strings or expression sets. The only difference is that in basic regular expressions the meta-characters ?, +, ' file.txt Alternation In GNU’s implementation of grep there is no functional difference between the basic and extended regular expression syntaxes. To interpret the pattern as an extended regular expression, use the -E ( or -extended-regexp) option. In its simplest form, when no regular expression type is given, grep interpret search patterns as basic regular expressions. GNU grep supports three regular expression syntaxes, Basic, Extended, and Perl-compatible. A pattern consists of operators, constructs literal characters, and meta-characters, which have special meaning. ![]() Grep Regular ExpressionĪ regular expression or regex is a pattern that matches a set of strings. In this article, we’re going to explore the basics of how to use regular expressions in the GNU version of grep, which is available by default in most Linux operating systems. grep searches one or more input files for lines that match a regular expression and writes each matching line to standard output. ![]() Grep is one of the most useful and powerful commands in Linux for text processing. Regular Expressions in Grep (Regex) With Examples Regular-Expressions-in-Grep-Regex.jpg
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