PHP Level 2, Regular Expressions

Explanation
The "+" symbol in the given statement means that there should be one or more characters between 0 and 9. This means that the regular expression will match any string that consists of one or more digits from 0 to 9.

Explanation
The given regular expression '/^[0-9]{3}-[0-9]{2,4}$/' matches strings that have the following pattern: three digits followed by a hyphen, followed by two to four digits. Therefore, the only string that matches this pattern is "213-99".

Explanation
strpos is fastest.

Explanation
The given regular expression requires a string that starts with one or more lowercase letters ([a-z]+), followed by zero or more digits ([0-9]*), and ends with 2 to 10 uppercase letters ([A-Z]{2,10}). In order to match this pattern, the string needs to have at least one lowercase letter, zero digits, and two uppercase letters. Therefore, the shortest string that would match this regex is "aAA", which has 3 characters.

Explanation
The value of $matches[1] is 323. This is because the preg_match function is used to search for a pattern in a string. In this case, the pattern is defined as '/([0-9]{3})-([0-9]{3})-([0-9]{4})/'. The string "TEL 323-555-1212" is searched for this pattern. The pattern consists of three groups, each enclosed in parentheses. The first group ([0-9]{3}) matches three digits, the second group matches another three digits, and the third group matches four digits. The matched values are stored in the $matches array, with $matches[1] representing the first group. Therefore, the value of $matches[1] is 323.

Explanation
Regular expressions in PHP are commonly used for validating input data, dividing input text into substrings, and replacing strings with other strings. However, converting hexadecimal into decimal is not a typical use for regular expressions in PHP. This task can be accomplished using built-in PHP functions or algorithms specifically designed for conversion between number systems.