In Regular expressions, optional quantifiers are denoted by a question mark ?. It indicates that a character can appear either 0 or 1 time. For example, the regular expression humou?r will match the text humour as well as the text humor.

In Regular expression, the literals are the simplest characters that will match the exact text of the literals. For example, the regex monkey will completely match the text monkey but will also match monkey in text The monkeys like to eat bananas.

In Regular expressions, fixed quantifiers are denoted by curly braces {}. It contains either the exact quantity or the quantity range of characters to be matched. For example, the regular expression roa{3}r will match the text roaaar, while the regular expression roa{3,6}r will match roaaar, roaaaar, roaaaaar, or roaaaaaar.

Alternation indicated by the pipe symbol |, allows for the matching of either of two subexpressions. For example, the regex baboons|gorillas will match the text baboons as well as the text gorillas.

Anchors (hat ^ and dollar sign $) are used in regular expressions to match text at the start and end of a string, respectively. For example, the regex ^Monkeys: my mortal enemy$ will completely match the text Monkeys: my mortal enemy but not match Spider Monkeys: my mortal enemy or Monkeys: my mortal enemy in the wild. The ^ ensures that the matched text begins with Monkeys, and the $ ensures the matched text ends with enemy.

Regular expressions are sequence of characters defining a pattern of text that needs to be found. They can be used for parsing the text files for specific pattern, verifying test results, and finding keywords in emails or webpages.

In Regular expression, wildcards are denoted with the period . and it can match any single character (letter, number, symbol or whitespace) in a piece of text. For example, the regular expression ......... will match the text orangutan, marsupial, or any other 9-character text.

Regular expression ranges are used to specify a range of characters that can be matched. Common regular expression ranges include: [A-Z]. : match any uppercase letter [a-z]. : match any lowercase letter [0-9]. : match any digit [A-Za-z] : match any uppercase or lowercase letter.

Shorthand character classes simplify writing regular expressions. For example, \w represents the regex range [A-Za-z0-9_], \d represents [0-9], \W represents [^A-Za-z0-9_] matching any character not included by \w, \D represents [^0-9] matching any character not included by \d.

In Regular expressions, the Kleene star(*) indicates that the preceding character can occur 0 or more times. For example, meo*w will match mew, meow, meooow, and meoooooooooooow. The Kleene plus(+) indicates that the preceding character can occur 1 or more times. For example, meo+w will match meow, meooow, and meoooooooooooow, but not match mew.

In Regular expressions, grouping is accomplished by open ( and close parenthesis ). Thus the regular expression I love (baboons|gorillas) will match the text I love baboons as well as I love gorillas, as the grouping limits the reach of the | to the text within the parentheses.

In PHP, filter_var is a function that filters a variable with a specified filter.

As its first argument, filter_var() takes a variable. As its second, it takes an ID representing the type of filtering that should be performed. The third argument is optional and can be used to specify options for the filter.

There are several filters for sanitizing common input types, including FILTER_SANITIZE_EMAIL, which is a common filter that removes illegal characters for an email address.

Filters can also be used to validate that text matches a pattern. For example, FILTER_VALIDATE_EMAIL returns the variable if it contains only valid email characters. Otherwise, it returns false.

In PHP, preg_match performs a regular expression match. The first argument is the pattern to match and the second argument is the variable to check. It returns 1 if it matches, 0 if it doesn’t, and FALSE if there was an error.

In PHP, strlen is a function that returns the length of the string passed in to it.

In PHP, preg_replace is a function that can replace portions of an input string based on a regular expression.

The first argument is the pattern to search for. The second argument is the new text to replace the pattern with. The third argument is the string to operate on.

The PHP header() function can be used to perform redirects.

We call the header() function on a string that begins with "Location: ", followed by the URL we want to redirect the user to. For example: "Location: https://www.codecademy.com/learn/learn-php/". After invoking the header() function we’ll want to use the language construct exit to terminate the current script.

In PHP, htmlspecialchars is a function that transforms special characters into HTML entities.

The built-in PHP trim() function is used to remove whitespace from the beginning and end of a string.