The attached patch for http://www.lyx.org/trac/ticket/7026 seems to be correct. The only thing: I don't understand why it should crash with isdigit() instead of iswdigit()...
Stephan
Index: src/support/lstrings.cpp =================================================================== --- src/support/lstrings.cpp (Revision 36309) +++ src/support/lstrings.cpp (Arbeitskopie) @@ -323,9 +323,7 @@ docstring::const_iterator cit = str.begin(); docstring::const_iterator const end = str.end(); for (; cit != end; ++cit) { - if (*cit == ' ') - continue; - if (isdigit((*cit))) + if (iswdigit((*cit))) return true; } return false;