REGULAR EXPRESSIONS IN PERL – PART I

William Fisher

Regular Expressions

A 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/; # matches

This 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 match

Regular Expressions

A variable can also be used in a regular expression.– $greeting = "World“; "Hello World" =~ /$greeting/; # matches

With 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'

Metacharacters

Metacharacters 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/; #

matches

Escape Sequences

Escape 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/ # matches

A 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/ # matches

Variables in Regular Expressions Variables 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/; # matches

Anchor Metacharacters

The ^ 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$/; # matches– "housekeeper\n" =~ /keeper$/; # matches

When both are used requires the entire string matches the parameters.– "keeper" =~ /^keep$/; # doesn't match– "keeper" =~ /^keeper$/; # matches– "" =~ /^$/; # ^$ matches an empty string

Character Classes

Character 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 way Another way to represent case-insensitivity

is the //i operator.– /yes/i

Special Characters and Range Operators in Character Classes Special 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 Class An ^ 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

ordinary

Common Character Classes

Certain 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 character

Word Anchor

The 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 Modifiers

The //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

line These 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 Metacharacter

The | 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"

Source

Kvale, Mark. Perl regular expressions tutorial. 2000 http://www.cs.drexel.edu/~knowak/cs265_fall_2009/perlretut_2007.pdf

Questions?