Regular Expressions in Perl – Part IRegular ExpressionsSlide 3Slide 4MetacharactersEscape SequencesVariables in Regular ExpressionsAnchor MetacharactersCharacter ClassesSpecial Characters and Range Operators in Character ClassesNegation in a Character ClassCommon Character ClassesWord Anchor//s and //m ModifiersAlternation MetacharacterSourceQuestions?REGULAR EXPRESSIONS IN PERL – PART IWilliam FisherRegular ExpressionsA regular expression is a string that represents a pattern.Can be used to search strings, extract desired parts of a string, or do a search and replace operation on a string.A basic regular expression uses the =~ operator as follows:–"Hello World" =~ /World/; # matchesThis returns a true if the string contains the desired pattern and false if it does not. The return is reversed if the ‘=‘ is replaced with an ‘!’.–"Hello World" !~ /World/; # does not matchRegular ExpressionsA variable can also be used in a regular expression.–$greeting = "World“; "Hello World" =~ /$greeting/; # matchesWith the default variable $_, $_ =~ can be omitted.–$_ = "Hello World";–if (/World/)The default delimiter can be replaced by putting an m in front of the expression. The default delimiter can then be used as a normal character.–"Hello World" =~ m!World!;Regular Expressions•Regular expressions match exactly to a string so they are case sensitive and consider ‘ ‘ to be a character.–"Hello World" =~ /world/; # doesn't match–"Hello World" =~ /oW/; # doesn't match–"Hello World" =~ /World /; # doesn't match•A regular expression also always matches the first instance of the pattern.–"That hat is red" =~ /hat/; # matches 'hat' in 'That'MetacharactersMetacharacters can be used to make more complicated matches. The meta characters are: {}[]()^$.|*+?\Metacharacters are treated as regular characters if preceded by a backslash."The interval is [0,1)." =~ /\[0,1\)\./ # matches"/usr/bin/perl" =~ /\/usr\/bin\/perl/; # matchesEscape SequencesEscape sequences are ASCII characters with no printable character equivilant such as \n, \t, \n ,\r, \a. They can be included in regular expressions just like any other character.–"1000\n2000" =~ /0\n20/ # matchesA backslash followed by three digits represents an octal number and a backslash followed by a lower case x (\x) and two digits (from 0 – F) represents a hexadecimal number.–"cat" =~ /\143\x61\x74/ # matchesVariables in Regular ExpressionsVariables can be included in regular expressions, similarly to how strings work with regular double quoted strings in Perl.$foo = 'house'; 'housecat' =~ /$foo/; # matches 'cathouse' =~ /cat$foo/; # matches 'housecat' =~ /${foo}cat/; # matchesAnchor MetacharactersThe ^ and $ metacharacters can be used to be used to require the expression to match at the beginning and end of a string respectively. The $ matches even if there is a /n, at the end of the string.–"housekeeper" =~ /^keeper/; # doesn't match–"housekeeper" =~ /keeper$/; # matches–"housekeeper\n" =~ /keeper$/; # matchesWhen both are used requires the entire string matches the parameters.–"keeper" =~ /^keep$/; # doesn't match–"keeper" =~ /^keeper$/; # matches–"" =~ /^$/; # ^$ matches an empty stringCharacter ClassesCharacter classes matches a set of possible characters, which are contained within brackets. […]–/[bcr]at/; # matches 'bat, 'cat', or 'rat'–/item[0123456789]/; # matches 'item0' or ... or 'item9‘–/[yY][eE][sS]/; # match 'yes' in a case-insensitive wayAnother way to represent case-insensitivity is the //i operator.–/yes/iSpecial Characters and Range Operators in Character ClassesSpecial characters can also be used in character classes as they are used in other places.–$x = 'bcr'; /[$x]at/; # matches 'bat', 'cat', or 'rat' /[\$x]at/; # matches '$at' or 'xat'The range operator ‘-’ can be used to represent contiguous sets characters as ranges such as [0-9] or [a-z]–/item[0-9]/; # matches 'item0' or ... or 'item9‘The range operator is treated as an ordinary character if it is at the beginning or end of the character class.Negation in a Character ClassAn ^ at the beginning of a class means that the character can be anything but what is included in the class./[^a]at/; # doesn't match 'aat' or 'at', but matches # all other 'bat', 'cat, '0at', '%at', etc./[^0-9]/; # matches a non-numeric character/[a^]at/; # matches 'aat' or '^at'; here '^' is ordinaryCommon Character ClassesCertain common character classes have abbreviations.–\d represents [0-9]–\s represents [\\\t\r\n\f] (whitespace character)–\w represents [0-9a-zA-Z]–‘.’ represents any character except \n\D, \S, and \W represent the negation of the character classes of their lower-case equivalents.These abbreviations can be used inside or outside of character classes.A period must be escaped or put in a character class to be used as a normal characterWord AnchorThe character \b matches a boundary between a word character and a non-ward character.\w\W or \W\w$x = "Housecat catenates house and cat"; $x =~ /cat/; # matches cat in 'housecat' $x =~ /\bcat/; # matches cat in 'catenates' $x =~ /cat\b/; # matches cat in 'housecat' $x =~ /\bcat\b/; # matches 'cat' at end of string//s and //m ModifiersThe //s modifier treats the string as a single line and therefore the ‘.’ character class will include \n.–$x = "There once was a girl\nWho programmed in Perl\n";–$x =~ /girl.Who/s; # matches, "." matches "\n"The //m modifier makes the anchor metacharacters treat each line as a new string so that the match can be at the beginning or end of any line.–$x =~ /^Who/m; # matches, "Who" at start of second lineThese modifiers can be combined (//sm) to get both of these effects.When using the //m modifier \A and \Z can still be used to match the beginning and the end of the string (ignoring the final \n) respectively. \z matches the end and considers the \n.Alternation MetacharacterThe | metacharacter can be used to match more than one possible string. –"cats and dogs" =~ /cat|dog|bird/; # matches “dog“The order of the string still predominates.–"cats and dogs" =~ /dog|cat|bird/; # matches "cat”In cases where more then one apply, the first one is used.–"cats" =~ /c|ca|cat|cats/; # matches "c"–"cats" =~ /cats|cat|ca|c/; # matches "cats"SourceKvale, Mark. Perl regular
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