This identifies uppercase alphabets that is from [A-Z].

    <?php
    $str = "AAAAbbbbcccc";
    $pat = ereg_replace("[[:upper:]]","a",$str);
    echo $pat;
    echo "\n<br>\n";
    ?>

    aaaabbbbcccc 

In the above example, the character class is used with ereg_replace. ereg_replace, replaces with string "a" as mentioned. Now,the regular expression identifies 'A' in the given string and ereg_replace replaces it with string "a" that is, in the string given "AAAAbbbbCCCC" having uppercase "A" with an lowercase "a".

    #! C:\programfiles\perl\bin\perl
    print "content-type: text/html\n\n";
    $str = "AAsfasaAA";
    $str =~ s|[[:upper:]]|X|g;
    print"Replaced String is $str"; 

    Replaced String is XXsfasaXX

In the above example the same pattern is used to search and replace operator in perl "s///" with a global scope "g" to replace every occurence of a string, so using the pattern "s|[[:upper:]]|X|g" every occurence of an uppercase alphabets is replaced by "X".