![]() Now that we’ve seen that PCRE is more powerful than BRE and ERE, we should note that not all grep implementations support the -P option. So next, let’s match lines containing “ Eric“, “ Kent“, and “ and” using grep -P: $ grep -P '(?=.*Kent)(?=.*Eric)(?=.*and)' input.txt However, if our grep supports the -P option, we can add more lookahead assertions to the pattern to solve the problem. Therefore, this approach isn’t ideal if we want to match more than two strings in the logical “And” relationship. Moreover, if we want to match four strings in the “And” scenario, we have 24 permutations, and five strings will lead to 120 permutations. If it contains both single and double quotes, enclose it in double quotes and prefix with a backslash the characters ',, and \: grep -E 'itsit'sletter \'e\'pay \20\a'C. Should your pattern include an apostrophe, enclose it in double quotes: grep -E 'itsit's'. Obviously, writing this pattern isn’t straightforward and it’s error-prone. Enclose your pattern in single quotes: grep -E 'aaabbb'. Then we have six permutations: ‘ A.*B.*C|A.*C.*B|B.*A.*C|B.*C.*A|C.*A.*B|C.*B.*A‘. grep awk sed grep multiple strings - syntax By default with grep with have -e argument which is used to grep a particular PATTERN. Let’s say we need to add one more string, ‘ C‘, to the matching list. Here, we put the permutations of two words in the pattern: ‘ A.*B|B.*A‘. It seems that the regular expression with | has no performance penalty.Let’s quickly revisit how we match two strings in the “And” scenario: ‘ Eric.*Kent|Kent.*Eric‘. Using -e option with grep command you can specify multiple patterns to match. Here we learn the different ways the grep command can be used to search for multiple strings or patterns in files. The clear winner is the hybring combining two positive greps and leaving the negative one in the pipe. The grep is a powerful tool in Linux for searching text, particularly when used with regular expressions. Awk and grep probably use the same regular expressions code and the logic of the two solutions is similar. Without parallelization - on a single core - the original pipe runs just slightly faster than awk which as a single process is not parallelized. The original pipe of three greps is pretty fast because of a good parallelization. pipe out to grep and feed data from grep. Here you see that the single grep is very slow because of the complex expression. The easiest way to put these sorts of expressions together is with multiple pipes. Hybrid - positive greps combined, negative piped $ time ( grep -E 'a.*c|c.*a' testfile | grep -v d >/dev/null ) The original three greps piped $ time ( grep a testfile | grep c | grep -v d >/dev/null ) Single awk $ time ( awk '/a/ & /c/ & $0 !~ /d/' testfile >/dev/null ) You can use Select-String similar to grep. ![]() Single grep $ time ( grep -E '^*a*c*$|^*c*a*$' testfile >/dev/null ) The Select-String cmdlet uses regular expression matching to search for text patterns in input strings and files. During the tests it was completely loaded in the cache so no disk operations affected the performance measurement. Using a simple program I have generated a random testing file with 200 000 000 lines, each with 4 characters as a random combination from characters a, b, c and d. Your original filter works very well and I think that in many cases the awk solution would be a little bit slower even on a single core. ![]() Because nowadays most of new computers have multiple CPU cores you can "naturally" utilize CPU parallelization just by using a pipe! Though the pipe communication is not the most effective possible but in most cases it is sufficient. Unix-like systems are designed to use pipes and to connect various utilities together. I think in most cases there is no reason for simplifying a pipe to a single command except when the combination results in a realatively simple grep expression which could be faster (see results below). ![]() Here is the equivalent of your filter: awk '/a/ & /c/ & $0 !~ /d/' If you just want to run a single command you can use awk which works with regular expressions too and can combine them with logical operators. Single command combination of the three greps The result has slow performance and the meaning of the expression is obscured. There are only complicated and ineffective ways. You cannot transform the filter grep a | grep c | grep -v d to a single simple grep. ![]()
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