Perl Basic 1

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my $str = "Hello saju how are you"  # Declaring and initializing a variable '$str'.

## Finding ##

$str =~ /pattern/          # Equivalent to function 'regmatch(pattern,$str)'.

Example:  $str =~ /saju/   # Finding for pattern 'saju' in variable '$str'.

## Finding and substituting or replacing ##

$str =~ s/pattern/replacement/  # Equivalent to function 'regsubstitute(pattern,replacement,$str)'.

Example: $str =~ s/saju/sanu/   # Finding for pattern 'saju' in variable '$str' and replace it with 'sanu'.

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# Regular expression is written in between / ... /.

# A few 11 characters mean something special in between / ... / , they are

  [], {}, (), *, +, ?, ., \, ^, $, | 

# Other characters in between / ... / just means themselves.

# \ ---> Backslash removing special meaning of special characters in between / ... / s

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### Character Classes ###

[aeiou] ---> Match any one of these characters a,e,i,o,u.

Example: /p[aeiou]t/  matches pat,pet,pit,pot,put.

Example: /p[^aeiou]t/ it not maches pat,pet,pit,pot,put.But it maches pbt,pct,pdt,----.

## character class shortcuts ##

[0123456789] ----> \d

[abc..xyxABC...XYZ0123456789_] ----> \w  # Except white space.

[ \n\r\t\f] ----> \s   # White space charater class.

[^0123456789] ----> \D

[^abc..xyxABC...XYZ0123456789_] ----> \W 

[^ \n\r\t\f] ----> \S   # opposite of White space charater class.

. ---> 'dot' shortcut matches any character(whilte space character,abc---xyzADC---XYZ0123456789_).It is widely used.

## character class shortcuts Examples ##

 /\d\d-\d\d-\d\d\d\d/  ---> 12-11-2009

 /\d\d\/\d\d\/\d\d\d\d/ ---> 11/10/2009

#or

 m{\d\d/\d\d/\d\d\d\d}  ---> 11/10/2009 # This method is used to avoid usage of delimiter Backslash '\' .Here we using m{..} sinstead of /../.

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### Quantifiers ###

# Normally, we match exactly one (thing)literal character or character class.

# Put a quantifier after a (thing) literal character or character class to change that.

# We can say we want to match

 1) Zero or one (thing) literal character or character class.  ---> ?

 2) Zero or more (thing) literal character or character class. ---> *

 3) One or more (thing) literal character or character class.  ---> +

## Zero or One thing --> ?   ##

/sa?ju/ ----> Matches 'saju', 'sju'. Here literal character 'a' matches Zero or One time. Not matches 'saaju'.

/s[aj]?u/ ----> Matches 'sju', 'sau', 'su'. Not matches 'saju'.Here characters in the character class '[aj]' matches Zero or One time,

## Zero or More thing --> *  ##

/sa*ju/ ----> Matches 'saju', 'sju', 'saaaju', etc. Here literal character 'a' matches Zero or More times.

/s[aj]*u/ ----> Matches 'saaaju', 'sjjju', 'saaajjju', 'su', 'saju', 'sajajaajju'.Here characters in the character class '[aj]' matches Zero or More times.

## One or More thing --> +   ##

/sa+ju/ ----> Matches 'saju', 'saaaju'. Not matches 'sju'. Here literal character 'a' must matches One or More times.

/s[aj]+u/ ----> Matches 'saju', 'saaaju', 'sajjju', 'saaajjju', sajajaajju'. Not matches 'su'.Here characters in the character class '[aj]' must matches One or More times.

## Without Qualifiers ##

/(s[aj]u)/ ----> Matches 'sau', 'sju'. Not matches 'saju','su'.

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### Anchors ###

\A  ---> Matches only at te begining of the text.It must be first thing in Regular Expression.

\Z  ---> MAtches only at the end, or newline followed by end. It must be last thing in Regular Expression.

Examples:

/\A\d+/ ---> Line start with digits.

/\s\d{5}\Z/ ----> Line end with space and 5 digits.

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### Capturing Groups ###

# Parentheses '()' capture whatever they match in $1,$2,----.

# Count left-most parentheses to get corresponding $n.

Example:

/\A(\w+),\s+([A-Z][A-Z])\s+(\d{5})\Z/  then,

print "City: $1 , State: $2 , Zip: $3 ";

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###  $_   ###

$_   ---> It is default variable for pattern matching and pattern substitution or replacement.

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### <> ###

Diamond operator or readline operator.

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chomp $str;   ---> function 'chomp' remove last character from a string if it is a newline '\n' character.

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