The regex house(cat|keeper) means match house followed by either cat or keeper. Parts of a regex are grouped by enclosing them in parentheses. The grouping metacharacters () allow a part of a regex to be treated as a single unit. # Grouping things and hierarchical matching Here, all the alternatives match at the first string position, so the first matches. "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. "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. A pattern consists of one or more character literals, operators, or constructs. At each character position, Perl will first try to match the first alternative, dog. A regular expression is a pattern that the regular expression engine attempts to match in input text. 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/ 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 Grouping things and hierarchical matching.Rather than start with technical details, we’ll start with a bunch of examples. The set of strings they are capable of matching goes way beyond what regular expressions from language theory can describe. Okay, in many programming languages, a regular expression is a pattern that matches strings or pieces of strings. regex, regexp, or r.e.), is a string that represents a regular (type-3) language. In formal language theory, a regular expression (a.k.a. Knowledge of regexes will allow you to save the day.
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