Detailed explanation of Java's regular expression to determine whether it is an integer, decimal or real number-javaTutorial-php.cn

经常会遇到这样的情况，需要判断一个字符串是否是一个合法的数，包括整数，小数或者实数。

网上查到很多文章大多是判断这个字符串是否全为数字，比如下面这段来自StringUtils的代码，可以看到，13.2这样的数字实际上会返回false，可是，他的确是一个数字。

public static boolean isNumeric(String str) {
  if (str == null) {
    return false;
  }
  int sz = str.length();
  for (int i = 0; i < sz; i++) {
    if (Character.isDigit(str.charAt(i)) == false) {
      return false;
    }
  }
  return true;
}

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当然，网上还能查到很多其他方式，诸如用正则表达式判断是否0-9，用字符ascii码判断是否是数字以及用Double.parseDouble()是否抛出异常来判断是否为数字。

事实上，除了最后一种方式能达到我们的要求，其他的都很难真正做到类似的判断。但是最后一种方式也很难区别出到底是正整数，负整数，正小数还是负小数，而且，捕获异常的方式实在是有些难看。

基于此原因，我自己写了一个工具类，专门用作数的检测，目前能够检测正整数，负整数，整数，正小数，负小数，小数以及实数，采用的仍然是正则表达式的方式，当然，如果有遗漏或者错误，欢迎联系我以便更正，同时也欢迎修改或使用这些代码以便符合你的应用场景。

可以简单讲下正则的思想以便修改，

1. 对于正整数而言，可以带+号，第一个数字不能为0

2. 对于负整数而言，必须带负号，第一个数字也不能为0

3. 对于整数而言，实际是由0，正整数和负整数组成的，所以偷个懒用前两个方法一起判断

4. 对于正小数而言，可以考带+号，并考虑两种情况，第一个数字为0和第一个数字不为0，第一个数字为0时，则小数点后面应该不为0，第一个数字不为0时，小数点后可以为任意数字

5. 对于负小数而言，必须带负号，其余都同上

6. 对于小数，可以带正负号，并且带小数点就行了，但是至少保证小数点有一边不为空，所以这里还是分左边不为空和右边不为空的情况

7. 实数比较简单，，要么是整数，要么是小数

package com.sap.cesp.creditinsight.web.app.util;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
  
public class NumberValidationUtils {
    
  private static boolean isMatch(String regex, String orginal){
    if (orginal == null || orginal.trim().equals("")) {
      return false;
    }
    Pattern pattern = Pattern.compile(regex);
    Matcher isNum = pattern.matcher(orginal);
    return isNum.matches();
  }
  
  public static boolean isPositiveInteger(String orginal) {
    return isMatch("^\\+{0,1}[1-9]\\d*", orginal);
  }
  
  public static boolean isNegativeInteger(String orginal) {
    return isMatch("^-[1-9]\\d*", orginal);
  }
  
  public static boolean isWholeNumber(String orginal) {
    return isMatch("[+-]{0,1}0", orginal) || isPositiveInteger(orginal) || isNegativeInteger(orginal);
  }
    
  public static boolean isPositiveDecimal(String orginal){
    return isMatch("\\+{0,1}[0]\\.[1-9]*|\\+{0,1}[1-9]\\d*\\.\\d*", orginal);
  }
    
  public static boolean isNegativeDecimal(String orginal){
    return isMatch("^-[0]\\.[1-9]*|^-[1-9]\\d*\\.\\d*", orginal);
  }
    
  public static boolean isDecimal(String orginal){
    return isMatch("[-+]{0,1}\\d+\\.\\d*|[-+]{0,1}\\d*\\.\\d+", orginal);
  }
    
  public static boolean isRealNumber(String orginal){
    return isWholeNumber(orginal) || isDecimal(orginal);
  }
  
}

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测试用例如下：

package com.sap.cesp.creditinsight.web.app.util;
  
import junit.framework.Assert;
  
import org.junit.Test;
  
public class NumberValidationUtilsTest {
  
  /**
   * Test method for {@link com.sap.cesp.creditinsight.web.app.util.NumberValidationUtils#isPositiveInteger(java.lang.String)}
   */
  //correct test case: 1, 87653521123567
  //wrong test case: 0.1, 0, 0123, -1, -0.1, ab
  @Test
  public void testIsPositiveInteger() {
    Assert.assertTrue(NumberValidationUtils.isPositiveInteger("1"));
    Assert.assertTrue(NumberValidationUtils.isPositiveInteger("+12"));
    Assert.assertTrue(NumberValidationUtils.isPositiveInteger("87653521123567"));
    Assert.assertFalse(NumberValidationUtils.isPositiveInteger("0.1"));
    Assert.assertFalse(NumberValidationUtils.isPositiveInteger("0"));
    Assert.assertFalse(NumberValidationUtils.isPositiveInteger("0123"));
    Assert.assertFalse(NumberValidationUtils.isPositiveInteger("-1"));
    Assert.assertFalse(NumberValidationUtils.isPositiveInteger("-0.1"));
    Assert.assertFalse(NumberValidationUtils.isPositiveInteger("ab"));
  }
  
  /**
   * Test method for {@link com.sap.cesp.creditinsight.web.app.util.NumberValidationUtils#isNegativeInteger(java.lang.String)}
   */
  //correct test case: -1, -87653521123567
  //wrong test case: 0.1, 0, 0123, 1, -0.1, -ab
  @Test
  public void testIsNegativeInteger() {
    Assert.assertTrue(NumberValidationUtils.isNegativeInteger("-1"));
    Assert.assertTrue(NumberValidationUtils.isNegativeInteger("-87653521123567"));
    Assert.assertFalse(NumberValidationUtils.isNegativeInteger("0.1"));
    Assert.assertFalse(NumberValidationUtils.isNegativeInteger("0"));
    Assert.assertFalse(NumberValidationUtils.isNegativeInteger("0123"));
    Assert.assertFalse(NumberValidationUtils.isNegativeInteger("1"));
    Assert.assertFalse(NumberValidationUtils.isNegativeInteger("-0.1"));
    Assert.assertFalse(NumberValidationUtils.isNegativeInteger("ab"));
  }
  
  /**
   * Test method for {@link com.sap.cesp.creditinsight.web.app.util.NumberValidationUtils#isWholeNumber(java.lang.String)}.
   */
  //correct test case: -1, 0, 1, 8673434231, -282464334
  //wrong test case: 0.1, 0123, -0.1, ab
  @Test
  public void testIsWholeNumber() {
    Assert.assertTrue(NumberValidationUtils.isWholeNumber("-1"));
    Assert.assertTrue(NumberValidationUtils.isWholeNumber("0"));
    Assert.assertTrue(NumberValidationUtils.isWholeNumber("1"));
    Assert.assertTrue(NumberValidationUtils.isWholeNumber("+12"));
    Assert.assertTrue(NumberValidationUtils.isWholeNumber("8673434231"));
    Assert.assertTrue(NumberValidationUtils.isWholeNumber("-282464334"));
    Assert.assertFalse(NumberValidationUtils.isWholeNumber("0123"));
    Assert.assertFalse(NumberValidationUtils.isWholeNumber("0.1"));
    Assert.assertFalse(NumberValidationUtils.isWholeNumber("-0.1"));
    Assert.assertFalse(NumberValidationUtils.isWholeNumber("ab"));
  }
  
  /**
   * Test method for {@link com.sap.cesp.creditinsight.web.app.util.NumberValidationUtils#isPositiveDecimal(java.lang.String)}
   */
  //correct test case: 0.1, 0.132213, 1.0
  //wrong test case: 1, 0.0, 0123, -1, -0.1
  @Test
  public void testIsPositiveDecimal() {
    Assert.assertTrue(NumberValidationUtils.isPositiveDecimal("0.1"));
    Assert.assertTrue(NumberValidationUtils.isPositiveDecimal("0.132213"));
    Assert.assertTrue(NumberValidationUtils.isPositiveDecimal("30.00"));
    Assert.assertTrue(NumberValidationUtils.isDecimal("0."));
    Assert.assertTrue(NumberValidationUtils.isPositiveDecimal("+12.0"));
    Assert.assertFalse(NumberValidationUtils.isPositiveDecimal("0123"));
    Assert.assertFalse(NumberValidationUtils.isPositiveDecimal("1"));
    Assert.assertFalse(NumberValidationUtils.isPositiveDecimal("0.0"));
    Assert.assertFalse(NumberValidationUtils.isPositiveDecimal("ab"));
    Assert.assertFalse(NumberValidationUtils.isPositiveDecimal("-1"));
    Assert.assertFalse(NumberValidationUtils.isPositiveDecimal("-0.1"));
  }
  
  /**
   * Test method for {@link com.sap.cesp.creditinsight.web.app.util.NumberValidationUtils#isNegativeDecimal(java.lang.String)}
   */
  //correct test case: -0.132213, -1.0
  //wrong test case: 1, 0, 0123, -1, 0.1
  @Test
  public void testIsNegativeDecimal() {
    Assert.assertTrue(NumberValidationUtils.isNegativeDecimal("-0.132213"));
    Assert.assertTrue(NumberValidationUtils.isNegativeDecimal("-1.0"));
    Assert.assertTrue(NumberValidationUtils.isDecimal("-0."));
    Assert.assertFalse(NumberValidationUtils.isNegativeDecimal("1"));
    Assert.assertFalse(NumberValidationUtils.isNegativeDecimal("0"));
    Assert.assertFalse(NumberValidationUtils.isNegativeDecimal("0123"));
    Assert.assertFalse(NumberValidationUtils.isNegativeDecimal("0.0"));
    Assert.assertFalse(NumberValidationUtils.isNegativeDecimal("ab"));
    Assert.assertFalse(NumberValidationUtils.isNegativeDecimal("-1"));
    Assert.assertFalse(NumberValidationUtils.isNegativeDecimal("0.1"));
  }
  
  /**
   * Test method for {@link com.sap.cesp.creditinsight.web.app.util.NumberValidationUtils#isDecimal(java.lang.String)}.
   */
  //correct test case: 0.1, 0.00, -0.132213
  //wrong test case: 1, 0, 0123, -1, 0., ba
  @Test
  public void testIsDecimal() {
    Assert.assertTrue(NumberValidationUtils.isDecimal("0.1"));
    Assert.assertTrue(NumberValidationUtils.isDecimal("0.00"));
    Assert.assertTrue(NumberValidationUtils.isDecimal("+0.0"));
    Assert.assertTrue(NumberValidationUtils.isDecimal("-0.132213"));
    Assert.assertTrue(NumberValidationUtils.isDecimal("0."));
    Assert.assertFalse(NumberValidationUtils.isDecimal("1"));
    Assert.assertFalse(NumberValidationUtils.isDecimal("0123"));
    Assert.assertFalse(NumberValidationUtils.isDecimal("0"));
    Assert.assertFalse(NumberValidationUtils.isDecimal("ab"));
    Assert.assertFalse(NumberValidationUtils.isDecimal("-1"));
      
  }
  
  /**
   * Test method for {@link com.sap.cesp.creditinsight.web.app.util.NumberValidationUtils#isRealNumber(java.lang.String)}.
   */
  //correct test case: 0.032213, -0.234, 0.0, 1, -1, 0
  //wrong test case: 00.13, ab, +0.14
  @Test
  public void testIsRealNumber() {
    Assert.assertTrue(NumberValidationUtils.isRealNumber("0.032213"));
    Assert.assertTrue(NumberValidationUtils.isRealNumber("-0.234"));
    Assert.assertTrue(NumberValidationUtils.isRealNumber("0.0"));
    Assert.assertTrue(NumberValidationUtils.isRealNumber("1"));
    Assert.assertTrue(NumberValidationUtils.isRealNumber("+0.14"));
    Assert.assertTrue(NumberValidationUtils.isRealNumber("-1"));
    Assert.assertTrue(NumberValidationUtils.isRealNumber("0.0"));
    Assert.assertFalse(NumberValidationUtils.isRealNumber("00.13"));
    Assert.assertFalse(NumberValidationUtils.isRealNumber("ab"));
      
  }
  
}

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