What is?! In regex?
It’s a negative lookahead, which means that for the expression to match, the part within (?!…) must not match. In this case the regex matches http:// only when it is not followed by the current host name (roughly, see Thilo’s comment).
What does an expression not contain?
An expression is a mathematical statement that does not contain an equal sign. It cannot be solved for unless the value of the variable is given. An equation is a mathematical statement or sentence comprised of two equalities or expressions joined with an equal sign.
Is it safe to use regex?
An attacker can then cause a program using a Regular Expression to enter these extreme situations and then hang for a very long time. Specifically with preg_match there is a known issue that can cause a PHP Segmentation Fault. So the answer is no, it is not safe because of issues such as these.
Does not contain operator?
Does Not Contain (!~) Operator. The result of a contains comparison is always True or False. If the pattern is found, a False is returned. If either operand is Null, then the result is Null.
How to write regular expressions?
three numeric characters\\d {3} OR|a left parenthesis\\(,followed by three digits\\d {3},followed by a close parenthesis\\),in a non-capturing group (?:)
What is a regular expression pattern?
A regular expression is a pattern of text that consists of ordinary characters (for example, letters a through z) and special characters, known as metacharacters .
What is a regular expression search?
Regular expression searching provides a way to search for advanced combinations of characters. A regular expression included in a search request must be quoted and must begin with ##.
What is regular expression matching?
Regular expressions are patterns used to match character combinations in strings. In JavaScript, regular expressions are also objects. These patterns are used with the exec and test methods of RegExp , and with the match, replace, search, and split methods of String.