The regular expression of Java learning

1 The concept of regular expressions

Regular Expressions (English: Regular expression, often abbreviated as regex in code).

A regular expression is a string that is described using a single string, used to define a matching rule, and matches a series of strings that conform to a certain syntactic rule. In development, regular expressions are often used to retrieve and replace text that conforms to a rule.

2 matching rules for regular expressions

Referring to the help document, there is a rule definition in the pattern class that has a regular expression, which explicitly distinguishes the uppercase and lowercase letters in the regular expression. Let's learn the rules of grammar.

Syntax rules for regular expressions:

Character: X

Meaning: The character x is represented

For example: The match rule is "a", then the string content that needs to be matched is "a"

Character: \ \

Meaning: The backslash character ' \ ' is represented

For example: The match rule is "\ \" , then the string content that needs to be matched is "\"

Character: \ t

Meaning: Tab

For example: The match rule is "\ t", then the corresponding effect is to create a tab space

Character: \ n

Meaning: line break

For example: The match rule is "\ n", then the corresponding effect is a newline, the next line of the cursor in the original position

Character: \ r

Meaning: Carriage return character

For example: The matching rule is "\ r" , then the corresponding effect is the effect of carriage return, the cursor to the next line beginning

Character class: [ABC]

Meaning: The character A, B, or C is represented

For example: The match rule is "[ABC]" , then the match is a character a, or a character B, or a character C

Character class: [^ABC]

Meaning: Any character other than a, B, or C is represented

For example: The match rule is "[^ABC]", then the match is not a character a, or is not a character B, or is not a character C any one character

Character class: [A-za-z]

Meaning: Represents a to Z or A to Z, and the letters at both ends are included

For example: The match rule is "[a-za-z]", then you need to match an uppercase or lowercase letter

Character class: [0-9]

Meaning: represents 0 to 9 digits, both ends of the number are included

For example: The match rule is "[0-9]", then you need to match a number

Character class: [A-za-z_0-9]

Meaning: A representation of a letter or number or an underscore (that is, a word character)

For example: The match rule is "[a-za-z_0-9]", then you need to match a letter or a number or a glide line

Predefined character classes:.

Meaning: Represents any character

For example, if the match rule is ".", then a match is required for an arbitrary character. If you want to use it. , use the matching rule "\ \." To achieve

Predefined character classes: \d

Meaning: represents 0 to 9 digits, both ends of the number are included, equivalent to [0-9]

For example, if the match rule is "\d", then it needs to match a number

Predefined character classes: \w

Meaning: A representation of a letter or number or an underscore (that is, a word character), equivalent to [a-za-z_0-9]

For example: The match rule is "\w", then it needs to match a letter or a number or a glide line

Boundary match: ^

Meaning: Represents the beginning of a line

For example, if the matching rule is ^[abc][0-9]$ , then the content that needs to be matched starts at [ABC], which is equivalent to the left double quotation mark

Boundary Match: $

Meaning: Represents the end of a line

For example, if the matching rule is ^[abc][0-9]$ , then the match must end with [0-9], which is equivalent to the right double quotation mark

Boundary Matching device: \b

Meaning: The word boundary is represented

For example: The match rule is "\b[abc]\b" , which means that the right and left sides of the letter A or B or C need non-word characters ([a-za-z_0-9])

Quantity Word: X?

Meaning: X appears once or once or no

For example: The match rule is "A?", then the content that needs to be matched is a character a, or a does not have

Quantity Word: x*

Meaning: X appears 0 or more times

For example, if the match rule is "A *" , then the content that needs to be matched is multiple characters A, or a does not

Quantity Word: x+

Meaning: X appears one or more times

For example: The match rule is "A +", then the content that needs to be matched is more than one character a, or a

Quantity Word: X{n}

Meaning: The x appears exactly n times

For example: The match rule is "a{5}", then the content that needs to match is 5 characters a

Quantity Word: X{n,}

What it means: X appears at least n times

For example: The match rule is "a{5,}", then the content that needs to be matched is a minimum of 5 characters a

Quantity Word: X{n,m}

Meaning: X appears at least n times, but not more than M

For example: The match rule is "a{5,8}", then the content that needs to match is 5 characters A to 8 characters a

3 Regular Expression rule-matching exercises

Please write a string that satisfies the following matching rules:

Rule: "[0-9]{6,12}"

What the rule needs to match is a number with a length of 6 to a digit .

Example: Using the data "123456789" to match the result is true;

Use the data "12345" to match the result to false.

Rule: "1[34578][0-9]{9}"

What this rule needs to match is: one- Digit mobile phone number, 1th digit 1, 2nd Digit 3, 4, 5, 7 , One of 8, and 9 digits from 0 to 9 for any number in the back.

such as: Using the data "12345678901" to match the result is false;

The match result with the data "13312345678" is true.

Rule: "A*b"

The rule needs to match the following: There is a Bafter multiple a or 0 a; b must be the last character.

such as: Using the data "Aaaaab" to match the result is true;

Use the data "ABC" to match the result to false.

1.4 Common methods in string classes involving regular expressions

L public boolean matches(string regex)//Determine if a string matches a given rule

Example: Check the QQ number.

1: The requirement must be 5-15 digits

2:0 cannot begin

Code Demo:

String QQ = "604154942";

String regex = "[1-9][0-9]{4,14}";

boolean Flag2 = qq.matches (regex);

Example: Verifying a mobile phone number

1: Required for 11 digits

2: The 1th bit is 1, the 2nd bit is 3, 4, 5, 7, 8, and the next 9 bits are any number between 0 and 9.

Code Demo:

String phone = "18800022116";

String regex = "1[34578][0-9]{9}";

boolean flag = phone.matches (regex);

L Public string[] split(string regex)//split this string based on a matching rule for a given regular expression

Example: Splitting a number in a string

Code Demo:

String s = "18-22-40-65";

String regex = "-";

String[] result = S.split (regex);

Code Demo:

String s = "18 22 40 65";

String regex = "";

String[] result = S.split (regex);

L public string replaceall(string regex,string Replacement)//replace the string contents of the rule with the new string

Example: replacing numbers in text with *

Code Demo:

String s = "hello12345world6789012";

String regex = "[0-9]";

String result = S.replaceall (regex, "*");

1.5 Regular Expression Exercises

L MATCH the correct number

Matching rules:

Match positive integer: "\\d+"

Match positive decimals: "\\d+\\.\\d+"

Match negative integer: "-\\d+"

Match negative decimals: "-\\d+\\.\\d+"

Match a positive number that retains two decimal places: "\\d+\\.\\d{2}"

Match a positive number that retains 1-3 decimal places: "\\d+\\.\\d{1,3}"

L Match a legitimate mailbox

Matching rules:

"[A-za-z_0-9][email protected][a-za-z_0-9]+ (\\.[ a-za-z_0-9]+) + "

"\\[email protected]\\w+ (\\.\\w+) +"

L get each number in the IP address (192.168.1.100)

Matching rules:

”\\.”

The regular expression of Java learning