The regex house(cat|keeper) means match house followed by either cat or keeper. All Perl programmers pass through a stage where they try to program everything as regexes and, when that’s not challenging enough, everything as a single. Parts of a regex are grouped by enclosing them in parentheses. Using grep is not cross-platform compatible, since -P / -perl-regexp is only available on GNU grep, not BSD grep. Regular expressions, or just regexes, are at the core of Perl’s text processing, and certainly are one of the features that made Perl so popular. The /d, /u, and /l modifiers are not likely to be of much use to you, and so you need not worry about them very much. The grouping metacharacters () allow a part of a regex to be treated as a single unit. /d, /u, /a, and /l, available starting in 5.14, are called the character set modifiers they affect the character set rules used for the regular expression. # Grouping things and hierarchical matching Here, all the alternatives match at the first string position, so the first matches. The -0777 makes it apply the regular expression to the whole thing instead of line by line. I did this in Cygwin: Perl can behave like grep or like sed. "cats" =~ /cats|cat|ca|c/ # matches "cats"Īt a given character position, the first alternative that allows the regex match to succeed will be the one that matches. Well, here's a wikipedia page for matching or replacing with Perl one liners. "cats and dogs" =~ /dog|cat|bird/ # matches "cat"Įven though dog is the first alternative in the second regex, cat is able to match earlier in the string. Some examples: "cats and dogs" =~ /cat|dog|bird/ # matches "cat" If cat doesn't match either, then the match fails and Perl moves to the next position in the string. If dog doesn't match, Perl will then try the next alternative, cat. matches any character except the newline. However, echo 'Hello Anne' grep -o 'A.' returns the expected Anne, since. For example, echo 'Hello Anne' grep -o 'A ' returns the string A. At each character position, Perl will first try to match the first alternative, dog. I wanted to point out that grep doesn't recognize the as a newline, so your first example only matches to the first n character. As before, Perl will try to match the regex at the earliest possible point in the string. To match dog or cat, we form the regex dog|cat. We can match different character strings with the alternation metacharacter '|'. "Hello World" =~ m /x # matches the whole string # Matching this or that If you're matching against $_, the $_ =~ part can be omitted: $_ = "Hello World" įinally, the // default delimiters for a match can be changed to arbitrary delimiters by putting an 'm' out front: "Hello World" =~ m!World! # matches, delimited by '!' Print "It matches\n" if "Hello World" =~ /$greeting/ The literal string in the regex can be replaced by a variable: $greeting = "World" The sense of the match can be reversed by using !~ operator: print "It doesn't match\n" if "Hello World" !~ /World/ This idea has several variations.Įxpressions like this are useful in conditionals: print "It matches\n" if "Hello World" =~ /World/ The /o option for regular expressions (documented in perlop and perlreref) tells Perl to compile the regular expression only once. In our case, World matches the second word in "Hello World", so the expression is true. The operator =~ associates the string with the regex match and produces a true value if the regex matched, or false if the regex did not match. In this statement, World is a regex and the // enclosing /World/ tells Perl to search a string for a match. A regex consisting of a word matches any string that contains that word: "Hello World" =~ /World/ # matches The simplest regex is simply a word, or more generally, a string of characters. This page assumes you already know things, like what a "pattern" is, and the basic syntax of using them. This page covers the very basics of understanding, creating and using regular expressions ('regexes') in Perl. Perlrequick - Perl regular expressions quick start #DESCRIPTION grep -o to get only the match instead of the whole line with matches.
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