st: counting the number of times a string appears in a string variable?

["Mingfeng Lin" <mingfeng.lin@gmail.com>]

Greetings,

I looked through the list of string functions but couldn't find one
that fits the bill.  Suppose I have a string variable X, and I would
like to generate a new numeric variable Y containing the number of
times a certain string appeared in X.  For instance

X = "johnabc johncd"

If I'd like to find the number of times "john" shows up in X, I hope to obtain
Y = 2

Is there a function in Stata to do this?

Thank you very much for your help!

Mingfeng
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