![]() The negative numbering starts with -1 to refer to the capture group closest to the backreference that was defined before the backreference. grep understands three different versions of regular expression syntax: basic (BRE), extended (ERE) and perl (PCRE). To reduce the number of results that are displayed, use the -m (max count) option. The line number for each matching line is displayed at the start of the line. To find characters a, b, c, and d, execute: egrep a-d mytext. The egrep command works with letter ranges as well. For instance, find numbers 2, 3, 4, or 5 with: egrep 2-5 mytext. # use '\g(?P+),(?P=col3),'Īnother useful approach when there are numerous capture groups is to use negative backreferences. You can make grep display the line number for each matching line by using the -n (line number) option. Executing egrep with a numbered range as the search string outputs all lines with specified numbers. Backreferences greater than \9 # no match as '\10' will be treated as '\1' and '0'.Backslash sequences inside character class # \w here matches \ and w.Character class metacharacters $ echo 'int a' | grep ''.# line anchors have to be always escaped to match literally ![]() Escaping metacharacters $ echo 'a^2 + b^2 - C*3' | grep 'b^2'.This section lists some of the them, along with examples. ![]() There are several subtle differences between PCRE and BRE/ERE for the same feature. This type of problem is usually solved using negative lookahead. The example_files directory has all the files used in the examples. See man pcrepattern or online manual for complete details. Only some of the commonly used features are presented in this chapter. PCRE is handy when you need advanced features like lookarounds, non-greedy quantifiers, possessive quantifiers, unicode character sets, subexpression calls and so on. PCRE is mostly similar, but not exactly the same as regular expressions present in the Perl programming language. The grep function evaluates the BLOCK or EXPR for each element of LIST, locally setting the variable equal to each element. The -P option will help you use Perl Compatible Regular Expressions (PCRE) instead of BRE/ERE. In this regard, Perls grep and map operators are worth mastering.
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