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Regular_expressions.md

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Regular Expressions

Regular expression look-up tables

Special characters Description
^ anchor, match from beginning of string
$ anchor, match end of string
. Match any character except newline character \n
| OR operator for matching multiple patterns
() for grouping patterns and also extraction
[] Character class to match one character among many
\^ use \ to match special characters like ^

Quantifiers Description
* Match zero or more times the preceding character
+ Match one or more times the preceding character
? Match zero or one times the preceding character
{n} Match exactly n times
{n,} Match atleast n times
{n,m} Match atleast n times but not more than m times

Character classes Description
[aeiou] Match any vowel
[^aeiou] ^ inverts selection, so this matches any consonant
[a-f] Match any of abcdef character
\d Match a digit, same as [0-9]
\D Match non-digit, same as [^0-9] or [^\d]
\w Match alphanumeric and underscore character, same as [a-zA-Z_]
\W Match non-alphanumeric and underscore character, same as [^a-zA-Z_] or [^\w]
\s Match white-space character, same as [\ \t\r\n\f]
\S Match non white-space character, same as [^\s]
\t Match horizontal tab character
\h Match horizontal space characters
\H Match other than horizontal space characters

Miscellaneous Description
\b word boundary
\B not a word boundary
\U uppercases pattern until end of string or \L or \E
\L lowercases pattern until end of string or \U or \E
\u uppercases only next character
\l lowercases only next character
\Q quote metacharacters until end of string or \E
\E terminate case conversion or quoted section
  • \u \U \l \L \Q \E will work in any double quoted strings
  • example: $book = "\uelantris" is equivalent to $book = "Elantris"

Modifier Description
g global, match all patterns in string, not just the first
i Ignore case
m multiline mode, ^ and $ anchors work on internal lines
s singleline mode, . will also match \n
e evaluate replacement string as an expression
r return substituted string without modifying the variable

Variable Description
$1, $2, $3 etc Matched patterns grouped inside ()
\1, \2, \3 etc backreferencing while defining match pattern
$` String before matched pattern
$& Matched pattern
$' String after matched pattern

Pattern matching and extraction

#!/usr/bin/perl
use strict;
use warnings;

my $string = "This is a sample string";

# positive and negative match examples
print "\$string: $string\n\n";
print "\$string has the pattern 'is'\n" if($string =~ m/is/);
print "\$string does not have the pattern 'this'\n" if($string !~ m/this/);
print "\$string has the pattern 'this' when matching case-insensitively\n\n" if($string =~ m/this/i);

# check out what happens when \b is not used
if($string =~ m/\bis/)
{
    # single quotes used to clarify space character in output
    print "Pattern specified:             '\\bis'\n";
    print "String before matched pattern: '$`'\n";
    print "String matching pattern:       '$&'\n";
    print "String after matched pattern:  '$''\n\n";
}

# assigning matched pattern to variables
my ($word1) = $string =~ m/(s[a-z]*e)/i;
print "Word starting with 's' and ending with 'e': $word1\n";
my (@all_words) = $string =~ m/([a-z]+)/gi;
print "All words in \$string: ";
print "'$_' " foreach(@all_words);
print "\n\n";

# what happens when output is saved in scalar context
# again, single quotes used to clarify output
my $match_output = $string =~ m/is/;
print "Output when pattern matches: '$match_output'\n";
$match_output = $string =~ m/line/;
print "Output when pattern does not match: '$match_output'\n";
  • General syntax for pattern matching: $var =~ m/PATTERN/MODIFIER
  • $var =~ m portion of the syntax can be ignored if matching against $_ variable
  • logically invert match pattern, use: $var !~ m/PATTERN/MODIFIER
  • when pattern doesn't match, undef value is returned which acts as a logical false condition
$ ./regex_match.pl
$string: This is a sample string

$string has the pattern 'is'
$string does not have the pattern 'this'
$string has the pattern 'this' when matching case-insensitively

Pattern specified:             '\bis'
String before matched pattern: 'This '
String matching pattern:       'is'
String after matched pattern:  ' a sample string'

Word starting with 's' and ending with 'e': sample
All words in $string: 'This' 'is' 'a' 'sample' 'string' 

Output when pattern matches: '1'
Output when pattern does not match: ''

Transliteration operator

#!/usr/bin/perl
use strict;
use warnings;

my $greeting = '===== Have a great day =====';
$greeting =~ tr/=/*/;
print "$greeting\n";

# s modifier lets you 'squash' repetitive character sequences
my $lose = "Don't loose hope :)";
$lose =~ tr/a-zA-Z//s;
print "$lose\n";

# d modifier is used to delete characters
# c modifier complements the searchlist
my $sentence = "Th'i1s is34 a; senten6#ce";
$sentence =~ tr/a-zA-Z //cd;
print "$sentence\n";

# r modifier lets you retain original variable untouched and assign modified string to another variable
my $uppercase_quote = 'SIMPLICITY IS THE ULTIMATE SOPHISTICATION';
my $lowercase_quote = $uppercase_quote =~ tr/A-Z/a-z/r;
print "$lowercase_quote\n";

# tr can be used to count matching characters
my $mixed_str  = 'He has 5 cricket bats, 2 sets of stumps and a glove set';
my $letter_cnt = $mixed_str =~ tr/a-zA-Z//;
my $digit_cnt  = $mixed_str =~ tr/0-9//;
print "\$mixed_str: $mixed_str\n";
print "\$mixed_str has $letter_cnt letters and $digit_cnt digits\n";
  • General syntax for transliteration: $var =~ tr/SEARCHLIST/REPLACEMENTLIST/MODIFIER
  • $var =~ portion of the syntax can be ignored for $_ variable
  • tr works by replacing SEARCHLIST by corresponding character in REPLACEMENTLIST
  • for modifiers d and s, REPLACEMENTLIST is optional
  • tr doesn't allow regular expression elements other than the range operator (ex: 0-9, a-f, etc)
$ ./regex_tr.pl
***** Have a great day *****
Don't lose hope :)
This is a sentence
simplicity is the ultimate sophistication
$mixed_str: He has 5 cricket bats, 2 sets of stumps and a glove set
$mixed_str has 40 letters and 2 digits

Search and Replace

#!/usr/bin/perl
use strict;
use warnings;

my $str = 'sample string';
$str =~ s/sample/test/;
print "simple replace:              $str\n";

# different delimiters like |,#,% etc can be used instead of /
my $date = '25/04/2016';
$date =~ s|/|-|g;
print "using different delimiter:   $date\n";

# use \ when you need to match special characters
my $greeting = '***** Have a great day *****';
$greeting =~ s/\*/=/g;
print "replacing special character: $greeting\n";

# pattern extraction
my $words = 'night and day';
$words =~ s/(\w+)( and )(\w+)/$3$2$1/;
print "swapping words:              $words\n\n";

# changing case of matched pattern
my $sentence = 'thIs iS a saMple StrIng';
print "\$sentence:     $sentence\n";
$sentence =~ s/^([a-z])(.*)/\U$1\L$2/;
print "Changing case: $sentence\n\n";

# e modifier
my $numbers = '34 28 91 42 5';
print "\$numbers:  $numbers\n";
$numbers =~ s/(\d+)/$1%5/ge;
print "numbers%5: $numbers\n\n";

# one can also call subroutines with e modifier
# use r modifier to retain original variable and assign output to another
$numbers = '1 2 3 4 5';
my $numbers_factorial = $numbers =~ s/(\d+)/num_fact($1)/ger;
print "\$numbers: $numbers\n";
print "numbers!: $numbers_factorial\n\n";

sub num_fact
{
    my ($num) = @_;
    return ($num == 0) ? 1 : $num * num_fact($num - 1);
}

# backreferencing in defining match pattern
my $line = 'Can you spot the the mistakes? I i seem to not';
print "\$line:     $line\n";
$line =~ s/\b(\w+) \1/$1/gi;
print "corrected: $line\n";
  • General syntax: $var =~ s/PATTERN/REPLACEMENTPATTERN/MODIFIER
  • $var =~ portion of the syntax can be ignored for $_ variable
$ ./regex_search_replace.pl
simple replace:              test string
using different delimiter:   25-04-2016
replacing special character: ===== Have a great day =====
swapping words:              day and night

$sentence:     thIs iS a saMple StrIng
Changing case: This is a sample string

$numbers:  34 28 91 42 5
numbers%5: 4 3 1 2 0

$numbers: 1 2 3 4 5
numbers!: 1 2 6 24 120

$line:     Can you spot the the mistakes? I i seem to not
corrected: Can you spot the mistakes? I seem to not

split

#!/usr/bin/perl
use strict;
use warnings;

# extracing words from a sentence
my $string = 'This is a sample string';
my @words = split /\s+/, $string;
print "\$string: $string\n";
print "words:   ".join(', ', @words)."\n\n";

# extracting letters from a word
my @letters = split //, $words[0];
print "splitting '$words[0]' into letters: @letters\n";

# saving even the pattern used to split
my $data = 'Rahul : 75 : 68 : 90';
my @columns = split /(\W+)/, $data;
print "\$data:    $data\n";
print "columns: ";
print "'$_' " foreach(@columns);
print "\n\n";

# using a count value to limit number of splits
my $info = '46 ways to publish a novel';
my @info_split = split /\s+/, $info, 2;
print "\$info:           $info\n";
print "\$info_split[0] = $info_split[0]\n";
print "\$info_split[1] = $info_split[1]\n";
  • General syntax for split: @output_var = split /PATTERN/, $var[, count]
  • , $var[, count] part of syntax can be ignored for $_ variable
$ ./regex_split.pl
$string: This is a sample string
words:   This, is, a, sample, string

splitting 'This' into letters: T h i s
$data:    Rahul : 75 : 68 : 90
columns: 'Rahul' ' : ' '75' ' : ' '68' ' : ' '90' 

$info:           46 ways to publish a novel
$info_split[0] = 46
$info_split[1] = ways to publish a novel

Further Reading