Regular Expressions Regex modifiers (flags) PCRE_EXTRA modifier
Example
A PCRE modifier that causes an error if any backslash in a pattern is followed by a letter that has no special meaning. By default, a backslash followed by a letter with no special meaning is treated as a literal.
E.g.
/big\y/
will match bigy, but
/big\y/X
will throw an exception.
Inline version: (?X)
