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POHow can I specify the priority of a match pattern in a Regex?
 

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  1. POHow can I specify the priority of a match pattern in a Regex?
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    <p>I'm writing a function-parsing engine which uses regular expressions to separate the individual terms (defined as a constant or a variable followed (optionally) by an operator). It's working great, except when I have grouped terms within other grouped terms. Here's the code I'm using:</p> <pre><code>//This matches an opening delimiter Regex openers = new Regex("[\\[\\{\\(]"); //This matches a closing delimiter Regex closers = new Regex("[\\]\\}\\)]"); //This matches the name of a variable (\w+) or a constant numeric value (\d+(\.\d+)?) Regex VariableOrConstant = new Regex("((\\d+(\\.\\d+)?)|\\w+)" + FunctionTerm.opRegex + "?"); //This matches the binary operators +, *, -, or / Regex ops = new Regex("[\\*\\+\\-/]"); //This compound Regex finds a single variable or constant term (including a proceeding operator, //if any) OR a group containing multiple terms (and their proceeding operators, if any) //and a proceeding operator, if any. //Matches that match this second pattern need to be added to the function as sub-functions, //not as individual terms, to ensure the correct evalutation order with parentheses. Regex splitter = new Regex( openers + "(" + VariableOrConstant + ")+" + closers + ops + "?" + "|" + "(" + VariableOrConstant + ")" + ops + "?"); </code></pre> <p>When "splitter" is matched against the string "4/(2*X*[2+1])", the matches' values are "4/", "2*", "X*", "2+", and "1", completely ignoring all of the delimiting parentheses and braces. I believe this is because the second half of the "splitter" Regex (the part after the "|") is being matched and overriding the other part of the pattern. This is bad- I want grouped expressions to take precedence over single terms. Does anyone know how I can do this? I looked into using positive/negative lookaheads and lookbehinds, but I'm honestly not sure how to use those, or what they're even for, for that matter, and I can't find any relevant examples... Thanks in advance.</p>
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    1. USMichael Hoffmann
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    1. COHave you looked into using something like Antlr? That's something more specialized for parsing / lexing.
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    2. COWhat is FunctionTerm.opRegex?
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      1. USRyan Pedersen
    3. COFunctionTerm.opRegex is the same as "ops"- it's used by the Parse method in my FunctionTerm class. I thought I replaced that with "ops" to simplify the code, but I guess I missed that one- sorry.
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      1. USMichael Hoffmann
 

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