In double quoted strings PHP tries to understand all kinds of escape
sequences, but \A is not one of them. Such sequences are left untouched,
including the backslash.
In single quoted strings, there are no escape sequences. All are left
untouched, which is what we want in case of a regular expression.
TL;DR: The resulting string is the same in both cases. I'm touching this
because my IDE shows a warning about the unknown \A escape sequence.
It must be either turned into "\\A" or '\A'.
Change-Id: Ie1e84c67c344faf77bc86a0b28dc82d31c3a7dbe
0c7bd7dbc8