string IndexBoundsBuilder::simpleRegex(....) {
  ....
  if (c == 'Q') {
    ....
  } else if ((c >= 'A' && c <= 'Z') || 
             (c >= 'a' && c <= 'z') || 
             (c >= '0' && c <= '0') ||          // <=
             (c == '\0')) {
    ....
  }
  ....
}

A link to the source code on GitHub

PVS-Studio warning: V590 Consider inspecting the 'c >= '0' && c <= '0'' expression. The expression is excessive or contains a misprint. index_bounds_builder.cpp 145

Most likely, the subexpression c >= '0' && c <= '0' has an error, there is no range check of the symbol (this subexpression will be true only in case c == '0'). Judging by other subexpressions, supposedly it should be as follows: c >= '0' && c <= '9'.

This issue was originally reported in SERVER-28570.