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Full Discussion: q with Perl Regex
Top Forums Shell Programming and Scripting q with Perl Regex Post 302217910 by JamesGoh on Wednesday 23rd of July 2008 11:18:29 PM
Old 07-24-2008
q with Perl Regex

For a programming exercise, I am mean to design a Perl script that detects double letters in a text file.

I tried the following expressions

Code:
# Check for any double letter within the alphabet

/[a-zA-Z]+/

# Check for any repetition of an alphanumeric character

/\w+/

Im aware that the + means to search for one or more occurences of that character, however trying both of these did not meet the requirements of my program.

Also

Code:
/[a-zA-Z]{1}/

did not prove to be helpful as well

After doing some searching, I stumbled across the correct form of the regex for the double letter case. It turned out to be

Code:
/(.)\1/

Now I know that . refers to any single character and the \1 refers to the first character in the line being read (if s/..../.... is being used), but Im still puzzled as to why /(.)\1/ works instead of /[a-zA-Z]+/ for the case of double letters ?

many thanks
James
 

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CTYPE_ALNUM(3)								 1							    CTYPE_ALNUM(3)

ctype_alnum - Check for alphanumeric character(s)

SYNOPSIS
bool ctype_alnum (string $text) DESCRIPTION
Checks if all of the characters in the provided string, $text, are alphanumeric. PARAMETERS
o $text - The tested string. RETURN VALUES
Returns TRUE if every character in $text is either a letter or a digit, FALSE otherwise. EXAMPLES
Example #1 A ctype_alnum(3) example (using the default locale) <?php $strings = array('AbCd1zyZ9', 'foo!#$bar'); foreach ($strings as $testcase) { if (ctype_alnum($testcase)) { echo "The string $testcase consists of all letters or digits. "; } else { echo "The string $testcase does not consist of all letters or digits. "; } } ?> The above example will output: The string AbCd1zyZ9 consists of all letters or digits. The string foo!#$bar does not consist of all letters or digits. NOTES
Note If an integer between -128 and 255 inclusive is provided, it is interpreted as the ASCII value of a single character (negative val- ues have 256 added in order to allow characters in the Extended ASCII range). Any other integer is interpreted as a string contain- ing the decimal digits of the integer. SEE ALSO
ctype_alpha(3), ctype_digit(3), setlocale(3). PHP Documentation Group CTYPE_ALNUM(3)
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