What potential issues can arise when trying to combine different regular expressions in PHP?

When combining different regular expressions in PHP, potential issues can arise due to conflicting patterns or unintended matching behavior. To solve this, it is important to carefully review and test the combined regular expressions to ensure they work as intended. One approach is to use named subpatterns to make the regular expressions more modular and easier to manage. 

// Example of combining regular expressions with named subpatterns
$regex1 = '/(?P<digits>\d+)/';
$regex2 = '/(?P<letters>[a-zA-Z]+)/';

$combinedRegex = $regex1 . $regex2;

// Test the combined regular expression
$string = '123abc';
if (preg_match($combinedRegex, $string, $matches)) {
    echo "Match found: " . $matches['digits'] . ' ' . $matches['letters'];
} else {
    echo "No match found";
}

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