Regex Tutorial - Quantifier "?" in Regular Expression.

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Regular-expression

Topic

Quantifier "?" in Regular Expression?

Explanation

The "?" metacharacter otherwise known as quantifier is used to find the character or a sequence of characters that may or may not occur.

For example if we have a pattern "lt?" it matches the string that has the letter "l" that may have a "t" or may not have a "t" followed by it. So the string matched would be "l","lt","looo", "looot" etc.

PHP Example:

    <?php
    $name = "ae";
    if (preg_match("/a(dca)?e/", $name))
     {echo "Pattern matches!";} 
    else 
     {echo "Pattern not matched!";}
    ?>

Result:

    Pattern matches!

In the above example possible matches can be "ae" or "adcae" only as "dca" is grouped using a "()".

Perl Example:

    #! C:\programfiles\perl\bin\perl
    print "content-type: text/html\n\n";
    if ("looo" =~ m/lt?/) 
      {print "The pattern matched.!\n";} 
    else 
      {print "The pattern not found!\n";}

Result:

    The pattern matched.!

In the above example's the pattern "lt?" means that the string should starts with "l" but doesnt have have a "t" following, even then the pattern is matched as "?" is used which means the preceding character "t" may occur or may not.

Non-greedy Pattern Matching:

This type of matching can be done using "?" along with the quantifiers "*", "?", "*" and "{}" to get the minimum matched characters.

Perl Example:

    $str= "exagerate";
    if ($str =~ /e(.*)e/)
     {print "The output from  a Greedy Match::$1";}
    print "<br>";
    if ($str =~ /e(.*?)e/)
     {print "The output from  a Non Greedy Match::$1";}

Result:

    The output from a Greedy Match::xagerat
    The output from a Non Greedy Match::xag

PHP Example:

    $name = "exagerate";
    preg_match("/e(.*)e/",$name,$matches);
    print "The Greedy Matched characters::";
    echo $matches[1];
    print "<br>";
    preg_match("/e(.*?)e/",$name,$matches);
    print "The Non Greedy Matched characters::";
    echo $matches[1];

Result:

    The Greedy Matched characters::xagerat
    The Non Greedy Matched characters::xag

In the above examples asyou can see in the second example that characters between the first and the next occurence of "e" is displayed and not the last "e".

How to use Quantifier "?" in Regular Expression.