![]() All other characters are considered as non-word characters. Word characters include alphanumeric characters (a-z, A-Z, and 0-9) and underscores (_). ![]() To return only those lines where the specified string is a whole word (enclosed by non-word characters), use the -w ( or -word-regexp) option: grep -w 'fatal\|error\|critical' /var/log/nginx/error.log So if you were searching for “error”, grep will also print the lines where “error” is embedded in larger words, such as “errorless” or “antiterrorists”. When searching for a string, grep will display all lines where the string is embedded in larger strings. To ignore case when searching, invoke grep with the -i option (or -ignore-case): grep -i 'fatal\|error\|critical' /var/log/nginx/error.log This means that the uppercase and lowercase characters are treated as distinct. Here is the same example using the extended regular expression, which eliminates the need to escape the operator | grep -E 'fatal|error|critical' /var/log/nginx/error.logīy default, grep is case sensitive. If the string you are searching includes spaces, enclose it in double quotation marks. In the following example, we are searching for all occurrences of the words fatal, error, and critical in the Nginx logĮrror file: grep 'fatal\|error\|critical' /var/log/nginx/error.log Literal strings are the most basic patterns. When using extended regular expression, do not escape the | operator: grep -E 'pattern1|pattern2' file.įor more information about how to construct regular expressions, check our article Grep regex To interpret the pattern as an extended regular expression, invoke grep the -E ( or -extended-regexp) option. This is why we are escaping the OR operator ( |) with a slash. To keep the special meanings of the meta-characters, they must be escaped with a backslash ( \). When using basic regular expressions, the meta-characters are interpreted as literal characters. The syntax for searching multiple patterns using the grep basic regular expressions is as follows: grep 'pattern1\|pattern2' file.Īlways enclose the regular expression in single quotes to avoid the interpretation and expansion of the meta-characters by the shell. 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. To search for multiple patterns, use the OR (alternation) operator. When no regular expression type is specified, grep interpret search patterns as basic regular expressions. GNU grep supports three regular expression syntaxes, Basic, Extended, and Perl-compatible. In this article, we’re going to show you how to use GNU grep to search for multiple strings or patterns. ![]() Turn takes precedence over alternation.Grep is a powerful command-line tool that allows you to searches one or more input files for lines that match a regular expression and writes each matching line to standard output. Repetition takes precedence over concatenation, which in Two regular expressions can be joined by the infix operator | the resulting regular expression matches any string Nating two substrings that respectively match the concatenated subexpressions. Regular expression matches any string formed by concate. Two regular expressions can be concatenated the resulting Multiple patterns are combined by or.-and -or -not ( ) Specify how multiple patterns are combined using Boolean expressions. : ? ^ _ ` The preceding item is matched at least n times, but not more than m times. This option has to be used for patterns starting with -and should be used in scripts passing user input to grep. Any char from the `' class, and any char not in the '' class. octal codes 000 through 037, or 'DEL' (octal 177) A range of characters can be specified by giving the first and last charac. List is the caret ^ then it matches any character not in Meaning can be quoted by preceding it with a backslash.Ī list of characters enclosed by matches any single character in that list if the first character of the ![]() Including all letters and digits, are regular expressions The fundamental building blocks are the regular expressions that match a single character. ![]() The following descriptionĪpplies to extended regular expressions differences forīasic regular expressions are summarized afterwards. In other implementations, basic regular expres. Is no difference in available functionality using either Grep understands two different versions of regular expression syntax: "basic" and "extended." In GNU grep, there To arithmetic expressions, by using various operators to Regular expressions are constructed analogously Regular ExpressionsĪ regular expression is a pattern that describes a set of Egrep is the same as grep -E.įgrep is the same as grep -F. Variant programs egrep and fgrep are available. By default, grep prints the matching lines. Grep searches the named input FILEs (or standard input if no filesĪre named, or the file name - is given) for lines containing a match to the Search input files for lines that match a given pattern. ![]()
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