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String replace with callback

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preg_replace_callback works by sending every matched capturing group to the defined callback and replaces it with the return value of the callback. This allows us to replace strings based on any kind of logic. 

$subject = "He said 123abc, I said 456efg, then she said 789hij";
$regex = "/\b(\d+)\w+/";

// This function replaces the matched entries conditionally 
// depending upon the first character of the capturing group
function regex_replace($matches){
    switch($matches[1][0]){
        case '7':
            $replacement = "<b>{$matches[0]}</b>";
            break;
        default:
            $replacement = "<i>{$matches[0]}</i>";
    }
    return $replacement;
}

$replaced_str = preg_replace_callback($regex, "regex_replace", $subject);

print_r($replaced_str);
# He said <i>123abc</i>, I said <i>456efg</i>, then she said <b>789hij</b>
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Regular expressions:
* Regular Expressions regexpPCRE
* Global RegExp match
* String matching with regular expressions
* Split string into array by a regular expression
* String replacing with regular expression
* String replace with callback
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