Description:
------------
OS: CentOS7
Source repo: remi
Version: PHP 7.0.30 (built: Apr 24 2018 21:28:23) ( NTS )

Reproduced both in cli and with web-server (as apache module).

Environment is pre-configured with bitrix-env (https://training.bitrix24.com/support/training/course/index.php?COURSE_ID=113&LESSON_ID=9463&LESSON_PATH=9681.9461.9463) on LXC container.

Test script:
---------------
<?php

$a = 'abc';
var_dump(preg_match('/^a/', $a));

$a = 'a\bc';
var_dump(preg_match('/^a\\b/', $a));

$a = '\abc';
var_dump(preg_match('/^\\a/', $a));

Expected result:
----------------
int(1)
int(1)
int(1)

Actual result:
--------------
int(1)
int(1)
int(0)