We obtain a necessary and sufficient condition on the Haar coefficients of a real function $f$ defined on $\mathbb{R}^+$ for the Lipschitz $α$ regularity of $f$ with respect to the ultrametric $δ(x,y)=\inf \{|I|: x, y\in I; I\in\mathcal{D}\}$, where $\mathcal{D}$ is the family of all dyadic intervals in $\mathbb{R}^+$ and $α$ is positive. Precisely, $f\in \textrm{Lip}_δ(α)$ if and only if $\left\vert\left\right\vert\leq C 2^{-(α+ \tfrac{1}{2})j}$, for some constant $C$, every $j\in\mathbb{Z}$ and every $k=0,1,2,\ldots$ Here, as usual $h^j_k(x)= 2^{j/2}h(2^jx-k)$ and $h(x)=\mathcal{X}_{[0,1/2)}(x)-\mathcal{X}_{[1/2,1)}(x)$.

6 pages. This manuscript has been accepted for publication in the Revista de la Uni\'{o}n Matem\'{a}tica Argentina