Need a regular expression for detecting simple math expressions

Question

I'm going to validate simple math expressions such these:

a = 1 + 2
b = 2.2
a = b + 5.5
a = b - -5.5
a = -1 + 2
a = -2.4
a = 3.5 / 0.2 + 1
a = 3 * -2.1

NOTE: Operator precedence is not important!

I try following expressions but i got nothing!!!

for digits: ^([-]|[.]|[-.]|[0-9])[0-9]*[.]*[0-9]+$
for operators: [-]|[+]|[*]|[/]
for variables: [a-z]+|[A-Z]+

I put these expressions in C# string variables and used .net Regex.Matches(...) to find matches. But i got nothing!

@Carl! i don't want a parser! I just want to validate some simple expressions described above. — Jalal, Oct 07 '10 at 17:05
@rerun: Yes, It's almost true but some times it's just Var = operand — Jalal, Oct 07 '10 at 17:06
Could be a useful link http://www.regexlib.com/DisplayPatterns.aspx — Gage, Oct 07 '10 at 17:07
@Jalal: Yes, you **do** want a parser. The *exact* intent of a parser is to validate expressions. — Jerry Coffin, Oct 07 '10 at 17:14

score 3 · Accepted Answer · edited May 23 '17 at 12:04

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This is not a great job for regular expressions. IF you want to evaluate the true mathematical expression you will not be able to come up with a regex that can handle all cases. As Carl Norum said it is a similar discussion as to why you can't parse html with regex.

edited May 23 '17 at 12:04

Community

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answered Oct 07 '10 at 16:56

rerun

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It's completely different by HTML! it's just a simple assignment expressions...! – Jalal Oct 07 '10 at 17:09
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If its not true math expression and you limit your input then sure. – rerun Oct 07 '10 at 17:14
I need validate some simple assignment expression! – Jalal Oct 08 '10 at 06:25

score 1 · Answer 2 · answered Oct 07 '10 at 17:03

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Try:

for digits: ((-[0-9]|[0-9])(\.[0-9])|[0-9])
for operators: (-|\+|\*|\/)
for variables: ([a-z]|[A-Z])

These looked fine when tested in Regulator.

EDIT

First one might be a little 'sloppy', I ran out of lunch time to mess with it :P

answered Oct 07 '10 at 17:03

Tony Abrams

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score 0 · Answer 3 · answered Oct 08 '10 at 03:27

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Writing good regular expressions for even simple things is harder than it seems. Start with proven expressions and modify as needed:

http://regexlib.com

If you can't find an example, they have a forum dedicated to exactly this kind of question.

answered Oct 08 '10 at 03:27

ErnieL

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Need a regular expression for detecting simple math expressions

3 Answers3