perl regular expressions

Regular Expressions

Regular Expression (or pattern) in Perl – is a template that either matches or doesn’t match a given string.

if( $str =~ /hello/){…

}

while( <STDIN> ){if( /hello/ ){…}

}

Regular Expressions in Perl:

@words = split /\s+/, $str;

Regular Expressions (3)

/to.*ols/ matches ‘to’, followed by any string, followed by ‘ols’./to?ols/ the character before ‘?’ is optional. Thus, there are only two matching strings – ‘tools’ and ‘tols’.

/hello.you/ matches any string that has ‘hello’, followed by any one (exactly one) character, followed by ‘you’./to*ols/ last character before ‘*’ may be repeated zero or more times. Matches ‘tools’,’tooooools’,’tols’ (but not ‘toxols’ !!!)/to+ols/ ------//------- one or more -----//------.

“.” matchs any char except a newline \n

Regular Expressions (4)

Grouping – parentheses ‘( )’ are used for grouping one or more characters.

/(tools)+/ matches “toolstoolstoolstools”.

Alternatives:/hello (world|Perl)/ - matches “hello world”, “hello Perl”.

Regular Expressions (5)

Character Class /Hello [abcde]/ matches “Hello a” or “Hello b” …/Hello [a-e]/ the same as above

Negating:[^abc] any char except a,b,c

Regular Expressions (6)

Shortcuts • \d digit• \w word character [A-Za-z0-9_]• \s white space

Negative ^ – [^\d] matches non digit \S anything not \s\D anything not \d

Regular Expressions (8)

Anchors^ - marks the beginning of the string $ - marks the end of the string

/^Hello Perl/ - matches “Hello Perl, good by Perl”, but not “Perl Hello Perl”

/^\s*$/ - matches all blank lines

/^abc/ - “^” beginning of a string/a\^bc/ - matches “\^” /[^abc]/ - negating

Regular Expressions (9)

\b - matches at either end of a word (matches the start or the end of a group of \w characters)

/\bPerl\b/ - matches “Hello Perl”, “Perl” but not “Perl++”

\B - negative of \b

Regular Expressions (10)

Backreferences:/(World|Perl) \1/ - matches “World World”, “Perl Perl”.

/((hello|hi) (world|Perl))/•\1 refers to (hello|hi) (world|Perl)•\2 refers to (hello|hi)•\3 refers to (world|Perl)

$1,$2,$3 store the values of \1,\2,\3 after a reg.expr. is applied.

Examples:

$date="12 10 10";if($date=~ /(\d+)/){ print $1.":".$2.":".$3.":\n"; #output ($2 and $3 are empty): #12:::}

if($date=~ /(\d+)(\s+\1)+/){ print $1.":".$2.":".$3.":\n"; #output (notice $3 is empty): #10: 10::}

$str="Hello World";if($str=~ /((Hello|Hi) (World|Perl))/){ print $1.":".$2.":".$3.":\n"; #output: #Hello World:Hello:World:}

$str="Hello Perl Hi";if($str=~ /((Hello|Hi) (World|Perl)) \1/){ print $1.":".$2.":".$3.":\n"; #output: non}

$str="Hi Perl Hi Perl";if($str=~ /((Hi|Hello) (World|Perl)) \1/){ print $1.":".$2.":".$3.":\n"; #output: #Hi Perl:Hi:Perl:}

Examples

1. What is it?/^0x[0-9a-fA-F]+$/

2. Date format: Month-Day-Year -> Year:Day:Month $date = “12-31-1901”;$date =~ s/(\d+)-(\d+)-(\d+)/$3:$2:$1/;

Examples

3. Make a pattern that matches any line of input that has the same word repeated two (or more) times in a row. Whitespace between words may differ.

Example

1. /\w+/ #matches a word2. /(\w+)/ #to remember later3. /(\w+)\1/ #two times4. /(\w+)\s+\1/ #whitespace between

words5. “This is a test” -> /\b(\w+)\s+\1/6. “This is the theory” -> /\b(\w+)\s+\1\b/

HomeWork

1) Write a regular expression that identifies a 24-hour clock. For example: 0:01, 00:20, 15:00, 23:59

2) Write a regular expression that identifies a floating point. For example: 10, 10.0001, -0.1, +001.3456789

For both assignments write a single program that identifies these patterns in the input lines and prints out only the matched patterns.

HomeWork

3) Write a CGI Perl script that extracts all http links from a given WWW page.

Input: http address. It is received from a HTML text box.Output: list of all http links found in <a href=“link”> field.

Input Examples:

http://www.tau.ac.ilhttp://www.cs.tau.ac.ilhttp://www.cnn.com

HomeWork (3)

Remarks:1) You need to create two pages - (1) html

page with a text box (2) cgi script that receives the input and formats output html file.

2) Unix command ‘wget’ downloads html files.

3) Use regular expressions. The code for parsing should be small, 3-10 lines.

Regular Expressions

Quantifiers:/a{3,6}/ - matches “a” repeated 3,4,5,6 times/(abc){3,}/ - matches three or more repetitions of “abc”./a{3}/ - matches exactly three repetitions of “a”.

* = {0,}+ = {1,}? = {0,1}

Negated Match

if( $str =~ /hello/){…

}

if( $str !~ /hello/){…

}

Negation

Regular Expressions (11)

$& - what really was matched $` - what was before$’ - the rest of the string after the matched pattern

$` . $& . $’ - original string

Regular Expressions (12)

Substitutions:s/T/U/; #substitutes T with U (only once)s/T/U/g; #global substitutions/\s+/ /g; #collapses whitespacess/(\w+) (\w+)/$2 $1/g;

s/T/U/; #applied on $_ variable$str =~ s/T/U/;

Regular Expressions (13)

File Extension Renaming:my ($from, $to) = @ARGV;@files = glob (“*.$from”);foreach $file (@files){

$newfile = $file;$newfile =~ s/\.$from/\.$to/g;rename($file, $newfile);

}

=~ s/\.$from$/\.$to/g

Split and Join

$str=“aaa bbb ccc dddd”;@words = split /\s+/, $str;$str = join ‘:‘, @words; #result is “aaa:bbb:ccc:dddd”

@words = split /\s+/, $_; “ aaa b” -> “”, “aaa”, “b”@words = split; “ aaa b” -> “aaa”, “b”@words = split ‘ ‘, $_; “ aaa b” -> “aaa”, “b”

Grep

grep EXPR, LIST;

@results = grep /^>/, @array;@results = grep /^>/, <FILE>;

Regular Expressions (2)

Regular Expressions in Unix:grep “include .*h” *.h

regular expression globes

Defined/Undef

my $i;

if( defined $i ) #false

$i=0;

if( defined $i ) #true

my %hash; #or %hash=(); defined %hash; #false, hash is empty

$hash{“1”}=“one”;exists($hash{“1”})==defined($hash{“1”})==true;

undef $hash{“1”};exists($hash{“1”})== true;defined($hash{“1”})==false;

delete $hash{“1”};exists($hash{“1”})== false;defined($hash{“1”})==false;