Regex: word boundary but for white space, beginning of line or end of line only

I am looking for some word boundary to cover those 3 cases:

1. beginning of string
2. end of string
3. white space

Is there something like that since \b covers also -,/ etc.?

Would like to replace \b in this pattern by something described above:

(\b\d*\sx\s|\b\d*x|\b)