![]() ![]() Is no difference in available functionality using either syntax. (.) grep understands two different versions of regular expression syntax: “basic” and “extended.” In GNU grep, there This can be used to specify multiple search patterns, or to protect a patternīeginning with a hyphen (-). Interpret PATTERN as an extended regular expression (ERE, see below). Selectively citing the man page of gnu-grep: -E, -extended-regexp ² while egrep has been deprecated by POSIX and is sometimes no longer found on some systems, on some other systems like Solaris when the POSIX or GNU utilities have not been installed, then egrep is your only option as its /bin/grep supports none of -e, -f, -E, \| or multi-line patterns ¹ some grep implementations support even more like perl-compatible ones with -P, or augmented ones with -X, -K for ksh wildcards. To work around that, with some grep implementations like GNU grep, you can use the -H option, or with any implementation, you can pass /dev/null as an extra argument. Note that if *.txt expands to a single file, grep won't prefix matching lines with its name like it does when there are more than one file. Or store those patterns in a file, one per line and run grep -f that-file - *.txt Or put patterns on several lines: grep - 'foo You can do this by preceding each pattern with the -e option. You need to pass the -E option to grep to select it (formerly that was done with the egrep separate command²) grep -E - 'foo|bar' *.txtĪnother possibility when you're just looking for any of several patterns (as opposed to building a complex pattern using disjunction) is to pass multiple patterns to grep. The portable way is to use the newer syntax, extended regular expressions. The old, default syntax ( basic regular expressions) doesn't support the alternation ( |) operator, though some versions have it as an extension, but written with a backslash. Second, grep supports at least¹ two syntaxes for patterns. If you do need a single quote, you can write it as '\'' (end string literal, literal quote, open string literal). (also note the - end-of-option-marker to stop some grep implementations including GNU grep from treating a file called -foo-.txt for instance (that would be expanded by the shell from *.txt) to be taken as an option (even though it follows a non-option argument here)). Single quotes prevent expansion of anything between them (including backslashes) the only thing you can't do then is have single quotes in the pattern. The easiest way to do that is to put single quotes around it. First, you need to protect the pattern from expansion by the shell. So in grep -Po '^\: \K.*', we do print the matching portion because of -o, but because of \K, that matching portion becomes the sequence of characters (. In recent PCREs, \K is an operator that resets the start of the matching portion. It also added the -P options to use perl compatible regular expressions (in PCRE) instead of basic ones without -P. While grep is just meant to print matching lines (is not otherwise a stream editor like sed is), GNU grep added the non-standard -o option for it to print the matching portion(s) of the line (if non-empty). So grep '^\: ' matches on lines that start with. ^ is another regular expression operator that means: match at the beginning of the line only. ![]() So you need to escape the opening ' or grep 'myword]'. is also a special operator in regular expressions (very similar to the glob operator), grep '' would match on lines that contain m, y, w, o, r or d. ![]() For instance, if there's a file called r in the current directory, and one called d, grep would become grep d r in all shells but fish. So they must be quoted for the shell (with single quotes for instance as in the solutions here). In grep, is expanded to the list of files that match that, that is any file in the current directory whose name is m, y, w, o, r or d (and if there's none, depending on the shell, the pattern is passed as-is to grep, or you get an error). In any case, note that in most shells, are glob operators. With GNU grep with support for recent PCRE, you can also do: grep -Po '^\: \K.*' < example.txt Better with sed: sed -n 's/^\: //p' < example.txt ![]()
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