Regular expression for arithmetic expression

Question

I,m trying to write a regex to check if the given string is like a + b, 2 + a + b, 3 + 6 * 9 + 6 * 5 + a * b, etc...

Only + and * operators.

I tried

if (str.matches("(\\d|\\w \\+|\\*){1,} \\d|\\w"))

Unfortunately it only handles cases like 3 * 7 ... (numeric * numeric).

Waiting for your answers, thanks for reading me.

The character class (ala @AvinashRaj) is clearly better. Purely for educational purposes the problem with your use of the vertical bar is that you didn't group the things before and after. If you wanted \\+|\\* then you needed to group them like (?:\\+|\\*). Same with the \\d|\\w although that's not so important since \\w includes \\d. — Peter Bowers
– Peter Bowers, Commented Apr 24, 2015 at 10:40
Matching operators (vs numerical constants / symbolic names) is only the first step of resolving an expression, called tokenizing. The next step is building a parse tree, so that you can properly evaluate parenthesis inside-out, multiplication before addition, etc. Related answer: stackoverflow.com/questions/62374663/… Blog on tokenizer & parse tree: blog.bitsrc.io/parsing-expressions-in-javascript-4c156f0cbaec — Peter Thoeny
– Peter Thoeny, Commented Jun 15, 2020 at 17:06

Avinash Raj · Accepted Answer · 2015-04-24 10:36:22Z

5

Put * and + inside a character class.

str.matches("\\w(?:\\s[+*]\\s\\w)+");

DEMO

answered Apr 24, 2015 at 10:36

Avinash Raj

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Avinash Raj Over a year ago

@MedAl since you're new to StackOverflow, i suggest you to read this

Steve Chaloner · Accepted Answer · 2015-04-24 10:39:59Z

2

This will handle cases of simple and chained calculations

[0-9A-Za-a]*( ){0,}([+-/*]( ){0,}[0-9A-Za-a]*( ){0,})*

This would match, for example

1+2
1 + 2
1 + a * 14 / 9

(You can change the operators you want by updating [+-/*])

answered Apr 24, 2015 at 10:39

Steve Chaloner

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4 Comments

Peter Bowers Over a year ago

Why all the {0,} instead of just *?

Peter Bowers Over a year ago

Since you have * instead of + after the [0-9a-z] your re will also match + or + - * / or etc

Steve Chaloner Over a year ago

It's ( ){0,} so it allows for optional spaces - and only spaces - between the operands

Peter Bowers Over a year ago

Yes, but space-star ( ` *` ) does the same thing as open-paren-space-close-paren-open-curly-zero-comma-close-curly. \s* would be even better.

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