How to split a string using regex, taking numbers with more of 1 digits #Java

Question

This program should be a calculator, so when the user insert a string like "6+9-2+4-12", it should give the result... The program does it right when the user gives only one-digit number like "6+2-1+3-2"; but if the user insert a 2-digit number like "12", the program consider it like 1 & 2.

This is the regex i used:

String[] somma = text1.split("(?<=[0-9]+)");

And this is the result of the expression "6+9-2+4-12":

6 + 9 - 2 + 4 - 1 2
Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: "+"
at java.base/jdk.internal.math.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:2054)
at java.base/jdk.internal.math.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
at java.base/java.lang.Double.parseDouble(Double.java:549)
at Calcolatrice.Ascoltatore.actionPerformed(Ascoltatore.java:69)
at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967)
at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308)
at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405)
at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262)
at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:279)
at java.desktop/java.awt.Component.processMouseEvent(Component.java:6636)
at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3342)
at java.desktop/java.awt.Component.processEvent(Component.java:6401)
at java.desktop/java.awt.Container.processEvent(Container.java:2263)
at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:5012)
at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2321)
at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844)
at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4919)
at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4548)
at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4489)
at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2307)
at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2764)
at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:772)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95)
at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:745)
at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:743)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:742)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

It should work because it takes numbers [0-9] of more digits with "+"... I don't know what to do anymore

Answer 1

One interpretation of the question is that you want to keep digits together, but split everything else into separate tokens, so you want to split:

• After a digit
• Before a digit
• Not between digits

As independent rules, that would be:

• After a digit that is not followed by a digit: (?<=\\d)(?!\\d)
• Before a digit is not preceded by a digit: (?<!\\d)(?=\\d)

So what you want is:

String[] somma = text1.split("(?<=\\d)(?!\\d)|(?<!\\d)(?=\\d)");

To not get an empty token at the beginning, we don't want to split "before a digit that is at the beginning of input", so we add that exact condition:

String[] somma = text1.split("(?<=\\d)(?!\\d)|(?<!\\d|^)(?=\\d)");
// result: ["6", "+", "9", "-", "2", "+", "4", "-", "12"]

This will allow evaluating an expression consisting of all operators, incl. parentheses, depending on the code that uses the result of this tokenization of the input expression.

---

Another interpretation of the question, based on the attempted split in the question, is that you only want to split after digits, so you want to split:

• After a digit
• Not between digits

That is of course even simpler:

String[] somma = text1.split("(?<=\\d)(?!\\d)");
// result: ["6", "+9", "-2", "+4", "-12"]

This will allow evaluating an expression consisting of only + and - operators, by simply parsing and adding the values.