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    <p>A full-blown parser for such a small grammar could be an overkill, specially when the OP obviously has no prior experience with them. Not even using parser generators like ANTLR or JavaCC seems a good idea.</p> <p>It's not easy to elaborate more with the current information. OP, please provide information requested as comments to your question.</p> <p>Tentative grammar:</p> <pre><code>maxExpr ::= maxExpr '|' '(' minExpr ')' maxExpr ::= '(' minExpr ')' minExpr ::= minExpr '&amp;' ITEM minExpr ::= ITEM ITEM ::= 'DDT\d{4}' </code></pre> <p>Realized that, true, the grammar is excessive for a RegEx, but for a <strong>single</strong> RegEx. Nobody is saying we can't use more than one. In fact, even the simplest RegEx substitution can be regarded as a step in a Turing machine, and thus the problem is solvable using them. So...</p> <pre><code>str= str.replaceAll("\\s+", "" ) ; str= str.replaceAll("&amp;", "," ) ; str= str.replaceAll("\\([^)]+\\)", "-$0" ) ; str= str.replaceAll("\\|", "," ) ; str= str.replaceAll(".+", "+($0)" ) ; str= str.replaceAll("\\w+", "x($0)" ) ; str= str.replaceAll("\\+", "max" ) ; str= str.replaceAll("-", "min" ) ; </code></pre> <p>I didn't take many shortcuts. The general idea is that "+" equates to a production of <code>maxExpr</code> and "-" to one of <code>minExpr</code>.</p> <p>I tested this with input</p> <pre><code>str= "(DDT1453 &amp; DDT1454 &amp; DDT1111) | (DDT3524 &amp; DDT3523 &amp; DDT3522 &amp; DDT3520)" ; </code></pre> <p>Output is:</p> <pre><code>max(min(x(DDT1453),x(DDT1454),x(DDT1111)),min(x(DDT3524),x(DDT3523),x(DDT3522),x(DDT3520))) </code></pre> <p>Back to the idea of a grammar, it's easy to recognize that the significant elements of it really are ITEMS and '|' . All the rest (parentheses and '&amp;') is just decoration.</p> <p>Simplified grammar:</p> <pre><code>maxExpr ::= maxExpr '|' minExpr maxExpr ::= minExpr minExpr ::= minExpr ITEM minExpr ::= ITEM ITEM ::= 'DDT\d{4}' </code></pre> <p>From here, a very simple finite automaton:</p> <pre><code>&lt;start&gt; maxExpr= new List() ; minExpr= new List() ; "Expecting ITEM" (BEFORE_ITEM): ITEM -&gt; minExpr.add(ITEM) ; move to "Expecting ITEM, |, or END" "Expecting ITEM, |, or END" (AFTER_ITEM): ITEM -&gt; minExpr.add(ITEM) ; move to "Expecting ITEM, |, or END" | -&gt; maxExpr.add(minExpr); minExpr= new List(); move to "Expecting ITEM" END -&gt; maxExpr.add(minExpr); move to &lt;finish&gt; </code></pre> <p>... and the corresponding implementation:</p> <pre><code>static Pattern pattern= Pattern.compile("(\\()|(\\))|(\\&amp;)|(\\|)|(\\w+)|(\\s+)") ; static enum TokenType { OPEN, CLOSE, MIN, MAX, ITEM, SPACE, _END_, _ERROR_ }; static enum State { BEFORE_ITEM, AFTER_ITEM, END } public static class Token { TokenType type; String value; public Token(TokenType type, String value) { this.type= type ; this.value= value ; } } public static class Lexer { Scanner scanner; public Lexer(String input) { this.scanner= new Scanner(input) ; } public Token getNext() { String tokenValue= scanner.findInLine(pattern) ; TokenType tokenType; if( tokenValue == null ) tokenType= TokenType._END_ ; else if( tokenValue.matches("\\s+") ) tokenType= TokenType.SPACE ; else if( "(".equals(tokenValue) ) tokenType= TokenType.OPEN ; else if( ")".equals(tokenValue) ) tokenType= TokenType.CLOSE ; else if( "&amp;".equals(tokenValue) ) tokenType= TokenType.MIN ; else if( "|".equals(tokenValue) ) tokenType= TokenType.MAX ; else if( tokenValue.matches("\\w+") ) tokenType= TokenType.ITEM ; else tokenType= TokenType._ERROR_ ; return new Token(tokenType,tokenValue) ; } public void close() { scanner.close(); } } public static String formatColl(String pre,Collection&lt;?&gt; coll,String sep,String post) { StringBuilder result= new StringBuilder() ; result.append(pre); boolean first= true ; for(Object item: coll ) { if( ! first ) result.append(sep); result.append(item); first= false ; } result.append(post); return result.toString() ; } public static void main(String... args) { String str= "(DDT1453 &amp; DDT1454) | (DDT3524 &amp; DDT3523 &amp; DDT3522 &amp; DDT3520)" ; Lexer lexer= new Lexer(str) ; State currentState= State.BEFORE_ITEM ; List&lt;List&lt;String&gt;&gt; maxExpr= new LinkedList&lt;List&lt;String&gt;&gt;() ; List&lt;String&gt; minExpr= new LinkedList&lt;String&gt;() ; while( currentState != State.END ) { Token token= lexer.getNext() ; switch( currentState ) { case BEFORE_ITEM: switch( token.type ) { case ITEM: minExpr.add("x("+token.value+")") ; currentState= State.AFTER_ITEM ; break; case _END_: maxExpr.add(minExpr) ; currentState= State.END ; break; default: // Ignore; preserve currentState, of course break; } break; case AFTER_ITEM: switch( token.type ) { case ITEM: minExpr.add("x("+token.value+")") ; currentState= State.AFTER_ITEM ; break; case MAX: maxExpr.add(minExpr) ; minExpr= new LinkedList&lt;String&gt;() ; currentState= State.BEFORE_ITEM ; break; case _END_: maxExpr.add(minExpr) ; currentState= State.END ; break; default: // Ignore; preserve currentState, of course break; } break; } } lexer.close(); System.out.println(maxExpr); List&lt;String&gt; maxResult= new LinkedList&lt;String&gt;() ; for(List&lt;String&gt; minItem: maxExpr ) { maxResult.add( formatColl("min(",minExpr,",",")") ) ; } System.out.println( formatColl("max(",maxResult,",",")") ); } </code></pre>
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