PHP Regular Expressions
A regular expression or regex is a sequence of characters that forms a search pattern. When you search for data in a text, you can use this search pattern to describe what you are searching for.
Regex are patterns for string matching, with special wildcards that can match entire portions of the target string.
Regular expressions are strings composed of delimiters, a pattern and optional modifiers.
PHP Regex Functions
this funtion returns 1 if the pattern was found in the string and 0 if not found
this funtion returns the number of times the pattern was found in the string, which may also be 0 if not found
this funtion returns a new string where matched patterns have been replaced with another string
this funtion is just like the explode funtion, but it uses regular expression to break or split text instead of literal string.