exchange char Z for S between vowels in a string

You can use two groups to check the existence of the vowels between Z , after they are captured, just play $1 (first group), $2 (second group) between S

$str = 'TEREZA , CEZAR, BRAZIL, ANZOL';
echo preg_replace('/(A|I|O|U|E)Z(A|I|O|U|E)/', '$1S$2', $str);

Via lookback

preg_replace('~(?<=[a-zA-Z])Z(?=[a-zA-Z])~', 'S', $str);

Explanation

• (?<=[a-zA-Z]) - will observe the ones that come before, but not capture.
• Z - literal character to be captured.
• (?=[a-zA-Z]) - will check what comes next but not capture.

Via group

preg_replace('~([a-zA-Z])Z([a-zA-Z])~', '$1S$2', $str);

Explanation

• ([a-zA-Z]) - group 1 that should be caught
• Z - literal character to be captured
• ([a-zA-Z]) - group 2 that should be caught
• $1S$2 - replaces what was captured in group1 + S + that was captured in group2

Using list of vowels:

$input = ['TEREZA' , 'CEZAR', 'BRAZIL', 'ANZOL'];
print_r(preg_replace('/([aeiou]+)z([aeiou]+)/i', '$1S$2', $input));

or:

echo preg_replace('/([aeiou]+)z([aeiou]+)/i', '$1S$2', 'BRAZIL');

Result:

  

Array ([0] => TERESA [1] => CESAR [2] => BRAZIL [3] => ANZOL)

     

BRAZIL