如何在Java中使用正则表达式在特定位置查找文本中的数字

Question

How to create a method which will find a number in String Text. I contain List of Strings which contain text like:

Radius of Circle is 7 cm
Rectangle 8 Height is 10 cm
Rectangle Width is 100 cm, Some text

Now I have to find all the number in these lines which are coming before cm so that I don't mistakenly find any other number.

How can it happen

Answer 1

A matching regular expression would be:

(\d+) cm

In order to obtain the captured number before the cm , you can use the Pattern and Matcher classes:

String line = "Radius of Circle is 7 cm";
Pattern pattern = Pattern.compile("(\\d+) cm");
Matcher matcher = pattern.matcher(line);
while (matcher.find()) {
    System.out.println("Value: " + matcher.group(1));
}

This example only matches the line from example (1), but can be easily repeated for each the lines contained in your list. See Java Regex Capture Groups for more information.

Answer 2

You have to find groups with only digits in the string using the following regex:

(?:\d{1,})

• \\d{1,} matches a digit (equal to [0-9])
• {1,} Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed
• (?:) The capturing group

Main##:

import java.util.regex.Pattern;  
import java.util.Scanner;  
import java.util.regex.Matcher;    
public class RegexExample{    
    public static void main(String[] args){    
        Scanner sc=new Scanner(System.in);  
        while (true) {    
            Pattern pattern = Pattern.compile("(?:\\d{1,})");    
            System.out.println("Enter text:");  
            Matcher matcher = pattern.matcher(sc.nextLine());    
            boolean found = false;    
            while (matcher.find()) {    
                System.out.println("I found the text "+matcher.group()+" starting at index "+    
                 matcher.start()+" and ending at index "+matcher.end());    
                found = true;    
            }    
            if(!found){    
                System.out.println("No match found.");    
            }    
        }    
    }    
} 

Example:

Enter text:
Radius of Circle is 7 cm
I found the text 7 starting at index 20 and ending at index 21
Enter text:
Rectangle 8 Height is 10 cm
I found the text 8 starting at index 10 and ending at index 11
I found the text 10 starting at index 22 and ending at index 24
Enter text:
Rectangle Width is 100 cm, Some text
I found the text 100 starting at index 19 and ending at index 22
Enter text:

Note: In java code, the character \\ it's an escape character. That's why you have to append another \\ .

Answer 3

The correct pattern to use here is:

(\\d+)\\s+cm\\b

For a one liner, we can try using String#replaceAll :

String input = "Rectangle Width is 100 cm, Some text";
String output = input.replaceAll(".*?(\\d+)\\s+cm\\b.*", "$1");
System.out.println(output);

Or, to find all matches in a given text, we can try using a formal pattern matcher:

String input = "Rectangle Width is 100 cm, Some text";
String pattern = "(\\d+)\\s+cm\\b";
Pattern r = Pattern.compile(pattern);
Matcher m = r.matcher(input);
while (m.find()) {
    System.out.println("Found measurement: " + m.group(1));
}