Parser for STL stereolithography data files
Well, it's not much of a mystery.  Your code is simple and pretty. Their code is ugly because it performs exactly what it needs to do, and no more:


} else if (!strncmp(sz,"vertex",6) &amp;&amp; ::IsSpaceOrNewLine(*(sz+6))) { // vertex 1.50000 1.50000 0.00000
    if (faceVertexCounter &gt;= 3) {
        ASSIMP_LOG_ERROR("STL: a facet with more than 3 vertices has been found");
        ++sz;
    } else {
        if (sz[6] == '\0') {
            throw DeadlyImportError("STL: unexpected EOF while parsing facet");
        }
        sz += 7;
        SkipSpaces(&amp;sz);
        positionBuffer.push_back(aiVector3D());
        aiVector3D* vn = &amp;positionBuffer.back();
        sz = fast_atoreal_move&lt;ai_real&gt;(sz, (ai_real&amp;)vn-&gt;x );
        SkipSpaces(&amp;sz);
        sz = fast_atoreal_move&lt;ai_real&gt;(sz, (ai_real&amp;)vn-&gt;y );
        SkipSpaces(&amp;sz);
        sz = fast_atoreal_move&lt;ai_real&gt;(sz, (ai_real&amp;)vn-&gt;z );
        faceVertexCounter++;
    }



A few differences in the amount of work it does:


There's no regex that needs to be compiled.
It's not splitting tokens into a list.  So, no list needs to be allocated and filled.  More importantly, no strings need to be allocated, copied, and destroyed.
The aiVector3D is allocated first, then each of the three numbers is parsed and written directly into the memory location of the coordinate, with no temporary variable assignments.


I'll also point out that your use of assert is improper.  You should only assert conditions that you know must be true due to logic.  You should not assert conditions that you hope to be true about the input.  If you compile with assertions disabled, then the validation code goes away!  That would introduce a bug, and possibly a security vulnerability.