The escape character in a regular expression is the backslash.
It's needed when the regular expression want to design a meta characters in a regular expression.
It
For example, if you want to match:
otherwise, they would be have be interpreted as meta-character
Php: Single and double quoted PHP strings have special meaning of backslash. Thus if \ has to be matched with a regular expression \\, then “\\\\” or '\\\\' must be used in PHP code.