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Regular expressions for float values and coordinates

  • Article

I was working with some pixel values the other day and I had this problem; I’m getting an RGBA value from my object model but it returns it on a string format (ie: 0, 1, 1, .5). For this specific task I only cared about the alpha value so I figured writing a regular expression for it should be the easiest way to do this and I was in for a fun surprise.

 

So I started out with this:

 

\d,\s\d,\s\d,\s\d

 

Wrong!! – I forgot one cardinal rule about regular expressions, they work against string content so a decimal will not be considered part of a number. So long story short what I thought should be a simple regular expression turned into a scary one (at some point I should have given up on it and used String.Split but where would the fun in that be?). So here’s the final version of it:

 

\d+(\.\d*)?,\s\d+(\.\d*)?,\s\d+(\.\d*)?,\s(?<alpha>\d+(\.\d*)?)

 

A breakdown of that expression

 

\d+(\.\d*)? – This will pick up my values, it will pick up both 1 and 1.0

,\s – This is the separator which in my case is always a comma followed by a space

(?<alpha>\d+(\.\d*)?) – This is the same as the first part except that I defined this group as alpha so that I could pull the value. I

 

And a snippet of the code where it would be used (I defined groups for all 4 values here:

Regex exp = new Regex(@"(?<red>\d(\.\d*)?),\s(?<green>\d(\.\d*)?),\s(?<blue>\d(\.\d*)?),\s(?<alpha>\d(\.\d*)?)");

// Get this value from somewhere

string value = ...

Match match = exp.Match(value);

string alpha = match.Result("${alpha}");

// Do some comparison with your alpha value …

Similar expressions can be used for coordinates (both 2D and 3D)

 

2D : (3.1,23,5) = \((?<X>\d(\.\d*)?),(?<Y>\d(\.\d*)?)\)

3D : (3.1,23,5,90) = \((?<X>\d(\.\d*)?),(?<Y>\d(\.\d*)?),(?<Z>\d(\.\d*)?)\)

Comments

  • Anonymous
    March 10, 2011
    oh，the article gives inspiration,I would picked it  up.Regular expressions  are boring me for a long time .