regex - Xml Schema Pattern At The Beginning Of A String -

i new xml schema , regex.

i want define pattern matches following structure:

<fantastic>:item.attribute</fantastic> 

as as:

<fantastic>:item.attribute,:item.attribute,:item.attribute</fantastic> 

i have pairs of items , attributes. if there more 1 pair, pairs should seperated comma. tell parser following:

'if pattern starts @ beginning of string no comma put before it. else comma needs put before (repeated) string.'

the pattern item , attribute more complicated. works exception of comma seperation between pairs.

i have done research , found out ^ used tell parser previous character has @ beginning of string.

but not use in xml schema pattern. exist in xml anyway?

how write this?

kind regards