Regular expression system tutorial (7)_PHP tutorial

7. Regular expression matching rules

　7.1 Basic pattern matching

Everything starts from the most basic. 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 strings that begin 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: pattern

once

and string

There once was a man from NewYork
Who kept all of his cash in a bucket.

is a match.

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:

　^

Similar, use It 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.

7.2 Character Clusters

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:

　[AaEeIiOoUu]

This pattern matches any vowel character, But it can only represent one character. Use a hyphen to represent a range of characters, such as:

[a-z] // Match all lowercase letters
[A-Z] // Match all uppercase letters
[a-zA-Z ] //Match all letters
　[0-9] //Match all numbers
　[0-9.-] //Match all numbers, periods and minus signs
　[ f ] //Match all white characters

Again, these only represent one character, which is very important. If you want to match a string consisting of a lowercase letter and a digit, such as "z2", "t6" or "g7", but not "ab2", "r2d3" or "b52", use this pattern:

　^[a-z][0-9]$

Although [a-z] represents a range of 26 letters, here it can only match strings where the first character is a lowercase letter .

It was mentioned earlier that ^ represents the beginning of a string, but it also has another meaning. When ^ is used within a set of square brackets, it means "not" or "exclude" and is often used to eliminate a certain character. Using the previous example, we require that the first character cannot be a number:

　^[^0-9][0-9]$

This pattern is the same as "&5", "g7 " and "-2" are matched, but "12" and "66" are not matched. Here are a few examples of excluding specific characters:

[^a-z] //All characters except lowercase letters
[^/^] //Except ()(/)(^) All characters
　[^"] //All characters except double quotes (") and single quotes ()

Special characters "." (dot, period) are used in regular expressions to represent all characters except "new line". So the pattern "^.5$" matches any two-character string that ends with the number 5 and begins with some other non-"newline" character. The pattern "." can match any string, except empty strings and strings containing only a "new line".

PHP regular expressions have some built-in universal character clusters, the list is as follows:

Character cluster meaning

[[:alpha:]] Any letter
[ [:digit:]] Any numbers
[[:alnum:]] Any letters and numbers
[[:space:]] Any white characters
[[:upper:]] Any uppercase letters
[[:lower:]] Any lowercase letter
[[:punct:]] Any punctuation mark
[[:xdigit:]] Any hexadecimal number, equivalent to [0-9a-fA- F]

7.3 Determine recurring

By now, you already know how to match a letter or number, but more often than not, you may want to match a word or a group of numbers.A word consists of several letters, and a group of numbers consists of several singular numbers. The curly braces ({}) following a character or character cluster are used to determine the number of times the preceding content is repeated.

Character cluster meaning
　^[a-zA-Z_]$ All letters and underscores
　^[[:alpha:]]{3}$ All 3-letter words
^a$ letter a
^a{4}$ aaaa
^a{2,4}$ aa, aaa or aaaa
^a{1,3}$ a, aa or aaa
^a{2,}$ A string containing more than two a's
^a{2,} Such as: aardvark and aaab, but not apple
a{2,} Such as: baad and aaa, but Nantucket does not work
{2} Two tab characters
.{2} All two characters

These examples describe three different uses of curly braces. A number, {x} means "the preceding character or character cluster appears only x times"; a number plus a comma, {x,} means "the preceding content appears x or more times"; two Comma-separated numbers, {x,y} means "the previous content appears at least x times, but not more than y times". We can extend the pattern to more words or numbers:

　^[a-zA-Z0-9_]{1,}$ //All strings containing more than one letter, number or underscore
　^[0-9]{1,}$ //All positive numbers
　^-{0,1}[0-9]{1,}$ //All integers
　^-{ 0,1}[0-9]{0,}.{0,1}[0-9]{0,}$ //All decimals

The last example is not easy to understand, is it? ? Look at it this way: with everything starting with an optional negative sign (-{0,1}) (^), followed by 0 or more digits ([0-9]{0,}), and an optional A decimal point (.{0,1}) followed by 0 or more digits ([0-9]{0,}) and nothing else ($). Below you will learn about the simpler methods you can use.

The special characters "?" are equal to {0,1}, they both represent: "0 or 1 previous content" or "the previous content is optional". So the example just now can be simplified to:

　^-?[0-9]{0,}.?[0-9]{0,}$

The special characters "*" and {0,} are equal, they both represent "0 or more previous contents". Finally, the character "+" is equal to {1,}, which means "1 or more previous contents", so the above 4 examples can be written as:

　^[a-zA-Z0- 9_]+$ //All strings containing more than one letter, number or underscore
　^[0-9]+$ //All positive numbers
　^-?[0-9]+$ / /All integers
　^-?[0-9]*.?[0-9]*$ //All decimals

　Of course this does not technically reduce the complexity of regular expressions , but can make them easier to read

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