$re = '/(?<=clients_field_)(.*)(?=:)/';
$str = '{
"success": true,
"companies" [
{
"clients_field_15bb4d92-0a64-4431-b9f6-d16873f808dd": "true",
"clients_field_4741a6c5-3855-4455-b487-0b38b0038ae6": "info@domain.com",
"clients_field_78f225e0-1a78-4930-b251-ad2217baeb1b": "2017-07-26",
"clients_field_e5b4ca25-363e-4421-8198-e446207c4fb0": "2017-07-28"
},
{
"clients_field_15bb4d92-0a64-4431-b9f6-d16873f808dd": "true",
"clients_field_4741a6c5-3855-4455-b487-0b38b0038ae6": "info@domain.com",
"clients_field_78f225e0-1a78-4930-b251-ad2217baeb1b": "2017-07-26",
"clients_field_e5b4ca25-363e-4421-8198-e446207c4fb0": "2017-07-28"
}
]
}';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php