PHP preg_match_all strings starting with C and length of 4 - php

Question

I'm attempting to search a text document with a PHP script and return all strings that start with the character "C" and have a length of 4. Some of the results will end in "=" but most will end in an alphanumeric character.
I was able to to successfully pull the ones that started with C and ended in =.
<?php
$str = file_get_contents('./FILENAME.txt', true);
preg_match_all('/C(.{2,3})=/', $str, $matches);
print_r($matches[0]);
foreach ($matches[0] as $sub)
{
$file = './CapturedData.txt';
file_put_contents($file, print_r("\"".$sub . "\"\n", true), FILE_APPEND);
}
?>
but when I tried to adjust it to pull all strings starting with C and ending in any alphanumeric character and being the length of 3/4, it just returns the first 3/4 characters out of ANY length strings.
I know I'm missing something simple, but its killing me. Anything I try just keeps returning the first x characters out of any length string. While I only want to return strings that have a length of 3/4, starting with "C" and ending in =,a-z,A-Z,0-9
EDIT:
Lets say these are the strings in the document:
blahblahblahCaa=
Cae=
CGGG
dontmatchthisCAAA
CAAAjkjkjk
XXXXXXCXXXX
I only want to return the 2nd and 3rd line

Answer 1

You must need to provide anchors to do an exact line match.
^C(?:\S{2,3})[a-zA-Z0-9=]$
DEMO
$input = <<<EOT
blahblahblahCaa=
Cae=
CGGG
dontmatchthisCAAA
CAAAjkjkjk
XXXXXXCXXXX
EOT;
preg_match_all("~^C(?:\S{2,3})[a-zA-Z0-9=]$~m", $input, $match);
print_r($match);
Output:
Array
(
[0] => Array
(
[0] => Cae=
[1] => CGGG
)
)
Regular Expression:
^ the beginning of the string
C 'C'
(?: group, but do not capture:
\S{2,3} non-whitespace (all but \n, \r, \t, \f,
and " ") (between 2 and 3 times)
) end of grouping
[a-zA-Z0-9=] any character of: 'a' to 'z', 'A' to 'Z',
'0' to '9', '='
$ before an optional \n, and the end of the
string

Answer 2

the comment above alludes to it -- your regex does not specify that the string has to end at the ending character, just that it has to contain it. So Caaaa matches, but so does Caaaabbb and so would Caaa=bbb. You don't say what your input format is, but if it's one word per line, you can match /^C(..|...)[a-zA-Z0-9=]$/m

Answer 3

From what i understands,
\bC\S{2,3}=?(?=\s)
Example : http://regex101.com/r/lR6kI3/3
C matches a C
\S{2,3} matches anything other than a space. {2,3} quantifies the regex 2 or 3 times
=? optional =
(?=\s) checks if the string is followed by space
Example usage
$re = "/\\bC\\S{2,3}=?(?=\\s)/m";
$str = "blahblahblahCaa= Cae= CGGG dontmatchthisCAAA CAAAjkjkjk XXXXXXCXXXX ";
preg_match_all($re, $str, $matches);
print_r($matches);
Will give an output as
Array ( [0] => Cae= [1] => CGGG )

Answer 4

try this:
$str = file_get_contents('FILENAME.txt', true);
// $str = "blahblahblahCaa=\nCae=\nCGGG\ndontmatchthisCAAA\nCAAAjkjkjk\nXXXXXXCXXXX";
preg_match_all('/^C[a-zA-Z0-9=]{3}$/m', $str, $matches);
var_dump($matches[0]);
foreach ($matches[0] as $sub)
{
$file = 'CapturedData.txt';
file_put_contents($file, $sub, FILE_APPEND);
}
live demo