<?php
$search = preg_quote('test');
$pattern = '#([^ ]+ )?'.$search.'( [^ ]+)?#';
$res = preg_match_all($pattern,'write your test data here',$regs);
var_dump($regs);
$res = preg_match_all($pattern,'test data here',$regs);
var_dump($regs);
$res = preg_match_all($pattern,'write your test',$regs);
var_dump($regs);
$res = preg_match_all($pattern,'not found at all',$regs);
var_dump($regs);
$res = preg_match_all($pattern,'write your test data found more than once test here ',$regs);
var_dump($regs);