Rename fracture piece @name attribute with padded numbering in Houdini VEX.

hou_padPieceNumber.h

// Fractured pieces are generated with @name such as 'piece0', 'piece1', etc.
// The following prim wrangle renames each piece with a padded number.

// String var to hold padded piece number.
string new_name = sprintf("piece_%04d", opdigits(@name));
// Update piece name.
s@name = new_name;