Question

I know strlen is used to determine the string length – but is it possible to search an entire block of text and pull out strings that are exactly 8 characters long?

Answer 1

The following assumes by “strings exactly eight characters long” you mean “words”, e.g. in a text like

$blockOfText = <<< TXT
codepad is an online compiler/interpreter, and a simple collaboration tool.
Paste your code below, and codepad will run it and give you a short URL you 
can use to share it in chat or email. 
TXT;

you want to find the word “compiler” only.

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Non-Regex solution (demo):

print_r(
    array_filter(
        str_word_count($blockOfText, 1),
        function($word) { return strlen($word) === 8; }
    )
);

Reference:

• str_word_count — Return information about words used in a string
• array_filter — Filters elements of an array using a callback function
• Callbacks
• Anonymous Functions

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Regex solution (demo):

preg_match_all(
    '(
        \b     # match from a word boundary
        \w{8}  # followed by exactly 8 "word" characters
        \b     # followed by a word boundary
    )x', 
    $blockOfText, 
    $matches
);
print_r($matches[0]);

A “word” character is any letter or digit or the underscore character, that is, any character which can be part of a Perl “word”. The definition of letters and digits is controlled by PCRE’s character tables, and may vary if locale-specific matching is taking place. For example, in the “fr” (French) locale, some character codes greater than 128 are used for accented letters, and these are matched by \w.

Reference:

• preg_match_all — Perform a global regular expression match
• PCRE Pattern Syntax