I test using this plugin together with ChildTicketsPlugin.
That plugin uses parent custom field values including a pound sign (#1234).
While the present code of this plugin handles that format upon _display_, it doesn't handle it when searching the database for the children of a given ticket.
The appended patch fixes that, by providing a new configuration option, parent_format, to use like this:
fields.parent = parent
; support ChildTicketsPlugin
parent_format = #%s
The option has a default %s which corresponds to the current use with plain ticket numbers in the custom field value.