为什么我的String数组长度是3而不是2？

Question

I'm trying to understand regex. I wanted to make a String[] using split to show me how many letters are in a given string expression?

import java.util.*; 
import java.io.*;

public class Main {

  public static String simpleSymbols(String str) {

     String result = "";
     String[] alpha = str.split("[\\+\\w\\+]");
     int alphaLength = alpha.length;
     // System.out.print(alphaLength);

     String[] charCount = str.split("[a-z]");
     int charCountLength = charCount.length;
     System.out.println(charCountLength);
   }
}

My input string is "+d+=3=+s+" . I split the string to count the number of letters in string. The array length should be two but I'm getting three. Also, I'm trying to make a regex to check the pattern +b+, with b being any letter in the alphabet? Is that correct?

Answer 1

So, a few things pop out to me:

First, your regex looks correct. If you're ever worried about how your regex will perform, you can use https://regexr.com/ to check it out. Just put your regex on the top and enter your string in the bottom to see if it is matching correctly

Second, upon close inspection, I see you're using the split function. While it is convenient for quickly splitting strings, you need to be careful as to what you are splitting on. In this case, you're removing all of the strings that you were initially looking at, which would make it impossible to find. If you print it out, you would notice that the following shows (for an input string of +d+=3=+s+):

+
+=3=+
+

Which shows that you accidentally cut out what you were looking to find in the first place. Now, there are several ways of fixing this, depending on what your criteria is.

Now, if what you wanted was just to separate on all +s and it doesn't matter that you find only what is directly bounded by +s, then split works awesome. Just do str.split("+") , and this will return you a list of the following (for +d+=3=+s+):

d
=3=
s

However, you can see that this poses a few problems. First, it doesn't strip out the =3= that we don't want, and second, it does not truly give us values that are surrounded by a +_+ format, where the underscore represents the string/char you're looking for.

Seeing as you're using +w, you intend to find words that are surrounded by +s. However, if you're just looking to find one character, I would suggest using another like [az] or [a-zA-Z] to be more specific. However, if you want to find multiple alphabetical characters, your pattern is fine. You can also add a * (0 or more) or a + (1 or more) at the end of the pattern to dictate what exactly you're looking for.

I won't give you the answer outright, but I'll give you a clue as to what to move towards. Try using a pattern and a matcher to find the regex that you listed above and then if you find a match, make sure to store it somewhere :)

Also, for future reference, you should always start a function name with a lower case, at least in Java. Only constants and class names should start in a capital :)

Answer 2

I am trying to use split to count the number of letters in that string. The array length should be two, but I'm getting three.

The regex in the split functions is used as delimiters and will not be shown in results. In your case "str.split([az])" means using alphabets as delimiters to separate your input string, which makes three substrings "(+)|d|(+=3=+)|s|(+)".

If you really want to count the number of letters using "split", use 'str.split("[^az]")'. But I would recommend using "java.util.regex.Matcher.find()" in order to find out all letters.

Also, I'm trying to make a regex to check the pattern +b+, with b being any letter in the alphabet? Is that correct?

Similarly, check the functions in "java.util.regex.Matcher".