Regular Expressions in Perl Part I

1. Regular Expressions in Perl Part I by Ayush Gupta

2. Simplest Regular Expression • The simplest regexp is simply a word, or more generally, a string of characters. • A regexp consisting of a word matches any string that contains that word: "Hello World" =~ /World/; # matches • The operator =~ associates the string with the regexp match and produces a true value if the regexp matched, or false if the regexp did not match.

3. Simple Word Matching • In the case in previous slide, “World” matches the second word in "Hello World", so the expression is true. Expressions like this are useful in conditionals: if ("Hello World" =~ /World/) { print "It matches\n"; } else { print "It doesn't match\n"; } • The literal string in the regexp can be replaced by variable, like, $greeting = "World"; • The if statement would then beif ("Hello World" =~ /$greeting/)

4. Simple Word Matching (cont’d) • If you're matching against the special default variable $_, the $_ =~ part can be omitted: $_ = "Hello World"; if (/World/) { print "It matches\n"; } else { print "It doesn't match\n"; } • 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 '!'

5. Match or No Match? • "Hello World" =~ /world/; It doesn't match because regexps are case-sensitive • "Hello World" =~ /o W/; # matches • "Hello World" =~ /oW/; It doesn't match because of a lack of a space character • "Hello World" =~ /World /; It doesn't match because there is a space at the end of the regexp, but not at the end of the string • Regular expressions must match a part of the string exactly in order for the statement to be true.

6. Metacharacters • These characters are reserved for use in regexp notation. • The metacharacters are {}[]()^$.|*+?\ • A metacharacter can be matched by putting a backslash before it:"2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter "2+2=4" =~ /2\+2/; # matches, \+ treated like an ordinary + • The backslash character '\' is a metacharacter itself and needs to be back slashed: 'C:\WIN32' =~ /C:\\WIN/; # matches

7. Escape Sequences • In addition to the metacharacters, there are some ASCII characters which don't have printable character equivalents. • Common examples are \t for a tab, \n for a newline, \r for a carriage return and \a for a bell."1000\t2000" =~ m(0\t2) # matches "1000\n2000" =~ /0\n20/ # matches "1000\t2000" =~ /\000\t2/ # doesn't match, "0" ne "\000"

8. AnchorMetacharacters • ^ and $ are the anchor metacharacters. • The anchor ^ means match at the beginning of the string • The anchor $ means match at the end of the string, or before a newline at the end of the string. "housekeeper" =~ /keeper/; # matches "housekeeper" =~ /^keeper/; # doesn't match "housekeeper" =~ /keeper$/; # matches "housekeeper\n" =~ /keeper$/; # matches • When both ^ and $ are used at the same time, the regexp has to match both the beginning and the end of the string, i.e., the regexp matches the whole string.

9. Character Class • A character class allows a set of possible characters, rather than just a single character, to match at a particular point in a regexp. • Character classes are denoted by brackets [...], with the set of characters to be possibly matched inside. /[bcr]at/; # matches 'bat, 'cat', or 'rat‘ • The special character '-' acts as a range operator within character classes, like, [0123456789] becomes [0-9] • The special character ^ in the first position of a character class denotes a negated character class, which matches any character but those in the brackets.

10. AlternationMetacharacter “|” • Enables our regexp to be able to match different possible words or character strings. • To match dog or cat, we form the regexp dog | cat. "cats and dogs" =~ /cat|dog|bird/; # matches "cat" "cats and dogs" =~ /dog|cat|bird/; # matches "cat” • Even though dog is the first alternative in the second regexp, cat is able to match earlier in the string.