Basic pattern matching
It all starts with the most basic. Patterns are the most basic elements of regular expressions. They are a set of characters that describe the characteristics of strings. The pattern can be very simple, composed of ordinary strings, or very complex, and often uses special characters to represent characters in a range, recurrence, or context. For example:
^once
This pattern contains a special character ^, indicating that the pattern matches only those strings starting with once. For example, this pattern matches the string "once upon a time" and does not match "There once was a man from NewYork". Just as the ^ symbol indicates the beginning, the $ symbol is used to match strings ending in a given pattern.
bucket$
This pattern matches "Who keep all of this cash in a bucket" and does not match "buckets". When the characters ^ and $ are used at the same time, they represent an exact match (strings are the same as patterns). For example:
^bucket$
Match only the string "bucket". If a pattern does not include ^ and $, then it matches any string containing the pattern. For example: mode
once
With string
There once was a man from NewYork Who kept all of his cash in a bucket.
It's a match.
The letters (o-n-c-e) in this pattern are literal characters, that is, they represent the letter itself, and the numbers are the same. Other slightly more complex characters, such as punctuation and white characters (spaces, tabs, etc.), use escape sequences. All escape sequences are headed with a backslash (\). The escape sequence of tab characters is: \t. So if we want to detect whether a string starts with a tab, we can use this pattern:
^\t
Similarly, \n means "new line" and \r means enter. Other special symbols can be used to add backslashes in front of them, such as the backslash itself is represented by \\, periods are represented by \., and so on.
Character cluster
In INTERNET programs, regular expressions are usually used to verify user input. After the user submits a FORM, it is not enough 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 the character cluster. To create a cluster of characters representing all vowel characters, put all vowel characters in a square bracket:
[AaEeIiOoUu]
This pattern matches any vowel character, but can only represent one character. A hyphen can represent a range of a character, such as:
[a-z] //Match all lowercase letters[A-Z] //Match all capital letters[a-zA-Z] //Match all letters[0-9] //Match all numbers[0-9\.\-] //Match all numbers, periods and minus signs[ \f\r\t\n] //Match all white characters
Similarly, these also represent only 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 a string whose first character is a lowercase letter.
It was mentioned earlier that ^ represents the beginning of a string, but it has another meaning. When using ^ in a set of square brackets, it means "non" or "excluded", and is often used to remove a character. Also using the previous example, we require that the first character cannot be a number:
^[^0-9][0-9]$
This pattern matches "&5", "g7" and "-2", but does not match "12" and "66". Here are a few examples of excluding specific characters:
[^a-z] //All characters except lowercase letters[^\\\/\^] //All characters except (\)(/)(^)[^\"\'] //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$" matches any two-character string ending with the number 5 and starting with other non-"newline" characters. Pattern "." can match any string except empty string and string that only includes one "new line".
PHP regular expressions have some built-in general character clusters, and the list is as follows:
| Character cluster | describe |
|---|---|
| [[:alpha:]] | Any letter |
| [[:digit:]] | Any number |
| [[:alnum:]] | Any letters and numbers |
| [[:space:]] | Any blank character |
| [[:upper:]] | Any capital letters |
| [[:lower:]] | Any lowercase letters |
| [[:punct:]] | Any punctuation mark |
| [[:xdigit:]] | Any hexadecimal number is equivalent to [0-9a-fA-F] |
Confirm to repeat
Until now, you have known how to match a letter or number, but more often, you might want to match a word or a set of numbers. A word consists of several letters, and a set of numbers consists of several singular numbers. Braces ({}) followed by characters or clusters of characters are used to determine the number of repetitions of the previous content.
| Character cluster | describe |
|---|---|
| ^[a-zA-Z_]$ | 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 |
| ^a{2,} | For example: aardvark and aaab, but apple cannot |
| a{2,} | For example: baad and aaa, but Nantucket cannot |
| \t{2} | Two tab characters |
| .{2} | All two characters |
These examples describe three different uses of curly braces. A number, {x} means "the previous character or character cluster only appears x times"; a number with a comma, {x,} means "the previous content appears x or more times"; two commas separated numbers, {x,y} means "the previous content appears at least x times, but no 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 all starting with an optional minus sign (\-{0,1}), followed by 0 or more numbers ([0-9]{0,}), and an optional decimal point (\.{0,1}) followed by 0 or more numbers ([0-9]{0,}), and nothing else ($). Below you will know the simpler methods that can be used.
The special characters "?" are equal to {0,1}, and they both represent: "0 or 1 previous content" or "the previous content is optional". So the example just now can be simplified to:
^\-?[0-9]{0,}\.?[0-9]{0,}$
The special characters "*" are equal to {0,}, and they both represent "0 or more previous contents". Finally, the character "+" is equal to {1,}, indicating "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.