Don't generate invalid anchor names.

HTML 4 anchor names have a number of restrictions. The analyzer
sometimes generates anchor names that break those restrictions, so
that the xref output doesn't fails the HTML validator. For now, skip
generating anchors for symbols that cause broken HTML.

Author: kahatlen

src/org/opensolaris/opengrok/analysis/JFlexXref.java

@@ -495,11 +495,19 @@ protected void writeSymbol(
             //    do this when there's exactly one definition of the symbol in
             //    this file? Otherwise, we may end up with multiple anchors with
             //    the same name.)
-            out.append("<a class=\"");
-            out.append(style_class);
-            out.append("\" name=\"");
-            out.append(symbol);
-            out.append("\"/>");
+            //
+            //    Note: In HTML 4, the name must start with a letter, and can
+            //    only contain letters, digits, hyphens, underscores, colons,
+            //    and periods. https://www.w3.org/TR/html4/types.html#type-name
+            //    Skip the anchor if the symbol name is not a valid anchor
+            //    name. This restriction is lifted in HTML 5.
+            if (symbol.matches("[a-zA-Z][a-zA-Z0-9_:.-]*")) {
+                out.append("<a class=\"");
+                out.append(style_class);
+                out.append("\" name=\"");
+                out.append(symbol);
+                out.append("\"/>");
+            }
 
             // 2) Create a link that searches for all references to this symbol.
             out.append("<a href=\"");

test/org/opensolaris/opengrok/analysis/haskell/sampleXrefExpected.html

@@ -6,5 +6,5 @@
 <a class="l" name="3" href="#3">3</a><a href="/source/s?defs=qsort" onmouseover="onMouseOverSymbol('qsort', 'undefined-in-file')">qsort</a> &#40;<a href="/source/s?defs=x" onmouseover="onMouseOverSymbol('x', 'undefined-in-file')">x</a>&#58;<a href="/source/s?defs=xs" onmouseover="onMouseOverSymbol('xs', 'undefined-in-file')">xs</a>&#41; &#61; <a href="/source/s?defs=qsort" onmouseover="onMouseOverSymbol('qsort', 'undefined-in-file')">qsort</a> &#91; <a href="/source/s?defs=x'" onmouseover="onMouseOverSymbol('x\'', 'undefined-in-file')">x'</a> &#124; <a href="/source/s?defs=x'" onmouseover="onMouseOverSymbol('x\'', 'undefined-in-file')">x'</a> &lt;&#45; <a href="/source/s?defs=xs" onmouseover="onMouseOverSymbol('xs', 'undefined-in-file')">xs</a>&#44; <a href="/source/s?defs=x'" onmouseover="onMouseOverSymbol('x\'', 'undefined-in-file')">x'</a> &lt; <a href="/source/s?defs=x" onmouseover="onMouseOverSymbol('x', 'undefined-in-file')">x</a> &#93; &#43;&#43; &#91;<a href="/source/s?defs=x" onmouseover="onMouseOverSymbol('x', 'undefined-in-file')">x</a>&#93; &#43;&#43; <a href="/source/s?defs=qsort" onmouseover="onMouseOverSymbol('qsort', 'undefined-in-file')">qsort</a> &#91; <a href="/source/s?defs=x'" onmouseover="onMouseOverSymbol('x\'', 'undefined-in-file')">x'</a> &#124; <a href="/source/s?defs=x'" onmouseover="onMouseOverSymbol('x\'', 'undefined-in-file')">x'</a> &lt;&#45; <a href="/source/s?defs=xs" onmouseover="onMouseOverSymbol('xs', 'undefined-in-file')">xs</a>&#44; <a href="/source/s?defs=x'" onmouseover="onMouseOverSymbol('x\'', 'undefined-in-file')">x'</a> &gt;&#61; <a href="/source/s?defs=x" onmouseover="onMouseOverSymbol('x', 'undefined-in-file')">x</a> &#93;
 <a class="l" name="4" href="#4">4</a>
 <a class="l" name="5" href="#5">5</a><span class="c">-- &#119;&#101;&#105;&#114;&#100; &#98;&#117;&#116; &#108;&#101;&#103;&#97;&#108; &#105;&#100;&#101;&#110;&#116;&#105;&#102;&#105;&#101;&#114;&#115;</span>
-<a class="l" name="6" href="#6">6</a><a class="xf" name="x'y'"/><a href="/source/s?refs=x'y'" class="xf" onmouseover="onMouseOverSymbol('x\'y\'', 'def')">x'y'</a> &#61; <b>let</b> <a href="/source/s?defs=f'" onmouseover="onMouseOverSymbol('f\'', 'undefined-in-file')">f'</a> &#61; <span class="n">1</span>&#59; <a href="/source/s?defs=g'h" onmouseover="onMouseOverSymbol('g\'h', 'undefined-in-file')">g'h</a> &#61; <span class="n">2</span> <b>in</b> <a href="/source/s?defs=f'" onmouseover="onMouseOverSymbol('f\'', 'undefined-in-file')">f'</a> &#43; <a href="/source/s?defs=g'h" onmouseover="onMouseOverSymbol('g\'h', 'undefined-in-file')">g'h</a>
+<a class="l" name="6" href="#6">6</a><a href="/source/s?refs=x'y'" class="xf" onmouseover="onMouseOverSymbol('x\'y\'', 'def')">x'y'</a> &#61; <b>let</b> <a href="/source/s?defs=f'" onmouseover="onMouseOverSymbol('f\'', 'undefined-in-file')">f'</a> &#61; <span class="n">1</span>&#59; <a href="/source/s?defs=g'h" onmouseover="onMouseOverSymbol('g\'h', 'undefined-in-file')">g'h</a> &#61; <span class="n">2</span> <b>in</b> <a href="/source/s?defs=f'" onmouseover="onMouseOverSymbol('f\'', 'undefined-in-file')">f'</a> &#43; <a href="/source/s?defs=g'h" onmouseover="onMouseOverSymbol('g\'h', 'undefined-in-file')">g'h</a>
 <a class="l" name="7" href="#7">7</a></pre></div></body></html>