如何使用preg_replace()仅替换特定行

您可以使用这样的正则表达式:

\n^(?=[a-z])

Working demo

$str = '123.- sumi hai
bla
124.- the
secod line
is blah
125.- another line';

$result = preg_replace('/\n^(?=[a-z])/m', ' ', $str);

echo "The result of the substitution is ".$result;