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copied!<p>You could try something like this:</p> <pre><code>function validateInfix(infix) { var balance = 0; // remove white spaces to simplify regex infix = infix.replace(/\s/g, ''); // if it has empty parenthesis then is not valid if (/\(\)/.test(infix)) { return false; } // valid values: integers and identifiers var value = '(\\d+|[a-zA-Z_]\\w*)'; // the unary '+' and '-' var unaryOper = '[\\+\\-]?'; // the arithmetic operators var arithOper = '[\\+\\-\\*\\/]'; // the comparison operators var compOper = '(\\<\\=?|\\>\\=?|\\=\\=|\\!\\=)'; // if it has more than one comparison operator then is not valid if (infix.match(new RegExp(compOper, 'g')).length > 1) { return false; } // the combined final regex: /[\+\-]?(\d+|[a-zA-Z_]\w*)(([\+\-\*\/]|(\<\=?|\>\=?|\=\=|\!\=))[\+\-]?(\d+|[a-zA-Z_]\w*))*/ var regex = new RegExp(unaryOper + value + '((' + arithOper + '|' + compOper + ')' + unaryOper + value + ')*'); // validate parenthesis balance for (var i = 0; i < infix.length; i++) { if (infix[i] == '(') { balance++; } else if (infix[i] == ')') { balance--; } if (balance < 0) { return false; } } if (balance > 0) { return false; } // remove all the parenthesis infix = infix.replace(/[\(\)]/g, ''); return regex.test(infix); } </code></pre> <p>The idea is to check first the parenthesis balance, then remove them all given that we only want to validate and not evaluate, and then match the remaining expression to a regex (which may not be perfect, I'm not a regex expert). And... just in case: <code>infix</code> argument must be a string.</p> <h2>Edit</h2> <p>I noticed a couple of details and changed the code a bit:</p> <ol> <li>Added the operators you needed the regex to match too.</li> <li>Removed white spaces to get rid of regex junk.</li> <li>Checked if the expression had empty parenthesis.</li> <li>Checked if the expression had more than one comparison operators.</li> <li>Changed this <code>\+?\-?</code> by this <code>[\+\-]?</code>.</li> <li>Changed <a href="http://www.w3schools.com/jsref/jsref_match.asp" rel="nofollow">string <code>match</code> method</a> by <a href="http://www.w3schools.com/jsref/jsref_regexp_test.asp" rel="nofollow">regex <code>test</code> method</a> where possible.</li> <li>Changed this <code>[a-zA-Z0-9]</code> by this <code>(\d+|[a-zA-Z_]\w*)</code> since the first one matches wrong identifiers like <code>53abc</code>.</li> <li>For better understanding and clarity, extracted pieces of regex into separate variables and built the final one from these.</li> </ol> <p>Hope this is ok for you now :)</p>

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