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How to create a progressive match regular expression?

I need a regular expression that matches a string when the user enters it. This is a little hard to explain, so let me show you what I mean:

It should match this line:

"XXXX units" , where XXXX is any number.

But it should also match any substring from the beginning of this line, therefore:

 "123" "123 u" "123 uni"

must also match.

But of course this should not match:

 "123 xx"

It seems so simple, but I can’t understand it. This is the closest I have:

 ^\d+ ?u?n?i?t?s?

... but this, unfortunately, also matches strings like "123us".

Can anyone help? This is javascript, so I can be limited a bit by the lack of appearance functions ...

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3 answers

Just add () :

 /^\d+( (u(n(i(t(s)?)?)?)?)?)?$/

Testing:

 /^\d+( (u(n(i(t(s)?)?)?)?)?)?$/.test("123 units") -> true /^\d+( (u(n(i(t(s)?)?)?)?)?)?$/.test("123 un") -> true /^\d+( (u(n(i(t(s)?)?)?)?)?)?$/.test("123 ui") -> false /^\d+( (u(n(i(t(s)?)?)?)?)?)?$/.test("12") -> true /^\d+( (u(n(i(t(s)?)?)?)?)?)?$/.test("123 xx") -> false
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It doesn't look great, but it does its job ...

 ^(\d+( (u(n(i(t(s)?)?)?)?)?)?)?$
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Demo here: https://regex101.com/r/uC7pX1/6

 /^\d+( (u(n(i(t(s)?)?)?)?)?)?$/
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