This paper addresses the problem of regularity properties of functions represented as an expansion in a wavelet basis with random coefficients in terms of finiteness of their Besov norm with probability 1. Such representations are used to specify a prior measure in Bayesian nonparametric wavelet regression. Investigating regularity of such functions is an important problem since the support of the posterior measure does not include functions that are not in the support of the prior measure, and therefore determines the functions that are possible to estimate using specified Bayesian model. We consider a more general parametrisation than has been studied previously which allows to study a priori regularity of functions under a wider class of priors. We also emphasise the difference between the abstract stochastic expansions that have been studied in the literature, and the expansions actually arising in nonparametric regression, and show that the latter cover a wider class of functions than the former. We also extend these results to stochastic expansions in an overcomplete wavelet dictionary.