Regular expressions, explained in great detail_PHP tutorial

Regular expressions in PHP (1)
PHP inherits the consistent tradition of *NIX and fully supports the processing of regular expressions. Regular expressions provide an advanced, but non-intuitive, method of string matching and processing. Friends who have used PERL's regular expressions know that regular expressions are very powerful, but they are not easy to learn.
For example:
^.+@.+..+$
This effective but incomprehensible code is enough to give some programmers a headache (me) or make them give up using regular expressions. I believe that after you finish reading this tutorial, you will understand the meaning of this code.
Basic Pattern Matching
Everything starts from the basics. Patterns are the most basic elements of regular expressions. They are a set of characters that describe the characteristics of a string. Patterns can be simple, consisting of ordinary strings, or very complex, often using special characters to represent a range of characters, recurrences, or to represent context. For example:
^once
This pattern contains a special character ^, which means that the pattern only matches those strings starting with once. For example, this pattern matches the string "once upon a time" but does not match "There once was a man from NewYork". Just like the ^ symbol indicates the beginning, the $ symbol matches strings that end with a given pattern.
bucket$
This pattern matches "Who kept all of this cash in a bucket", but does not match "buckets". When the characters ^ and $ are used together, they represent an exact match (strings are the same as patterns). For example:
^bucket$
only matches the string "bucket". If a pattern does not include ^ and $, then it matches any string that contains the pattern. For example: the pattern
once
matches the string
There once was a man from NewYork
Who kept all of his cash in a bucket.
.
The letters (o-n-c-e) in this pattern are literal characters, that is, they represent the letters themselves, and the same goes for numbers. Other slightly more complex characters, such as punctuation and white characters (spaces, tabs, etc.), require escape sequences. All escape sequences begin with a backslash (). The escape sequence for the tab character is: . So if we want to detect whether a string starts with a tab character, we can use this pattern:
^
Similarly, use means "new line" and means carriage return. Other special symbols can be used with a backslash in front, such as the backslash itself is represented by ., the period is represented by ., and so on.
Character cluster
In INTERNET programs, regular expressions are usually used to verify user input. When a user submits a FORM, it is not enough to use ordinary literal characters to determine whether the entered phone number, address, email address, credit card number, etc. are valid.
So we need to use a more free way to describe the pattern we want, which is character clusters. To create a cluster representing all vowel characters, place all vowel characters in square brackets:

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