在 Java 中，Regex 或正規表示式是一種應用程式介面，可協助定義搜尋、操作和編輯字串的模式。 Java 正規表示式廣泛用於密碼和電子郵件的驗證。這些表達式由 java.util.regex 套件提供，由 1 個介面和 3 個類別組成。
這三個類別是：
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Java Regex 有一個稱為 MatchResultInterface 的接口，可協助確定正規表示式的匹配操作結果。
讓我們看看如何借助程式用 Java 寫正規表示式。
代碼：
//Java program to demonstrate regular expressions
import java.util.regex.*;
public class RegExamples {
public static void main(String args[]){
String A = " Happiness is " + " within yourself";
String B = ".*within.*";
// checks whether the string A contains the word 'within' and stores the result in matchcheck
boolean matchcheck = Pattern.matches(B, A);
//prints the result
System.out.println("Is there any string 'within' in the text ? \n " + matchcheck);
} }輸出：
正規表示式中常用的方法有3種。
索引方法提供索引值，有助於精確顯示在作為輸入給出的字串中找到匹配項的位置。
| Method | Description |
| start() | The previous match’s start index is returned. |
| start(int group) | Given the group’s previous match operation, the subsequence is captured and returned. |
| end() | The offset after matching the last character is returned. |
| End(int group) | Given the group’s previous match operation, subsequence is captured and offset after matching its last character returned. |
| Method | Description |
| lookingAt() | Match the sequence given as input against the pattern from the beginning of the region. |
| find() | Finds the next subsequence of the sequence given as input against the pattern from the beginning of the region. |
| find(int start) | Resets the matcher and then finds the next subsequence of the sequence given as input against the specified index pattern. |
| matches() | Matches content against the pattern. |
方法
| Method | Description |
| appendReplacement(StringBuffer s, String replacement) | A non-terminal append and replacement step will be implemented. |
| appendTail(StringBuffer s) | A terminal append and replacement step will be implemented. |
| replaceAll(String replacement) | Replace all subsequence of the sequence given as input that matches against the pattern with a replacement string. |
| quoteReplacement(String s) | A literal replacement string will be returned for the mentioned string. |
| replaceFirst(String replacement) | Replace the first subsequence of the sequence given as input that matches the pattern with a replacement string. |
There are several ways in which a regular expression can be defined.
Suppose a string “hai” has to be searched in the text “hai”.
It can be done using syntax.
Pattern.matches("hai", "hai")It matches every single character in the text given as input against multiple permitted characters in the character class.
The following are the various class constructs.
| Character Class | Explanation |
| [pqr] | Matches the text if it contains either p, q or r, and it should be only once. |
| [^pqr] | ^ denotes the negation, and due to that, here, single character except for p, q, or r are taken. |
| [a-zA-Z] | a to z and A to Z are considered. |
| [a-d[p-s]] | a to d, or p to s. |
| [a-dm-p] | Union of both ranges. |
| [a-z&&[pqr]] | a to z and (p, q or r). |
| [a-z&&[^pq]] | a to z and also, p, q are not considered. |
| [ad-z] | Performs the subtraction. |
| [a-z&&[^m-p]] | a to z and not m to p. |
Metacharacters act like shortcodes in the regular expression.
The following are some of the metacharacters commonly used.
| Regular Expression | Explanation |
| \d | Any digit from 0 to 9. It can be written as [0-9] as well. |
| \D | Any non-digit from 0 to 9. It can be written as [^0-9] as well. |
| \s | Whitespace character or [\t\n\x0B\f\r]. |
| \S | Non whitespace character or [^\s]. |
| \w | Word character or [a-zA-Z_0-9]. |
| \W | Non-word character or [^\w]. |
| \b | Word boundary. |
| \B | Non-word boundary. |
Quantifiers mention the count of occurrence of each character to match against the string.
| Regular Expression | Explanation |
| a? | It occurs once or not at all. |
| A* | A occurs 0 or more times. |
| A+ | A occurs 1 or more times. |
| A{n} | A occurs exactly n times. |
| A{n,} | A occurs n or more than that. |
| A{n,m} | A occurs at least n times, but it should not be more than m times. |
Now, let us see a java program with the above-mentioned regular expressions.
Code:
//Java program to demonstrate regular expressions
import java.util.regex.*;
public class RegExamples {
public static void main(String args[]){
String str="hai";
// Returns true if string 1 matches string 2
System.out.println("Returns true if 'hai' matches 'Hai' :"+
Pattern.matches(str, "Hai")); //False
//Returns true if Hai or hai matches parameter 2
System.out.println("Returns true if 'Hai' or 'hai' matches 'Hai' : "+
Pattern.matches("[Hh]ai", "Hai")); //True
// Returns true if the string matches exactly "ann" or "Ann" or "jak" or "Jak"
System.out.println("Returns true if the string matches exactly 'ann' or 'Ann' or 'jak' or 'Jak' with 'Ann' : "+
Pattern.matches("[aA]nn|[jJ]ak", "Ann"));//True
//returns true if the string contains "with" at any place in the string
System.out.println("returns true if the string contains 'with' in the string 'within' : " +
Pattern.matches(".*with.*", "within"));//True
// returns true if the '9448anna' does not have number in the beginning
System.out.println( "returns true if the '9448anna' does not have number in the beginning : "+
Pattern.matches("^[^\\d].*", "9448anna")); //False
System.out.println("returns true if the '9448anna' does not have number in the beginning : " +
Pattern.matches("^[^\\d].*", "anna9448")); //True
}
}Output:
Java Regular Expressions are widely used for real-time applications such as password and email verification. These expressions are APIs that define patterns and offer searching, editing, and several other operations in the string.
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