Regular concatenation grammar

Retain every production $\alpha \to \beta$ of $P$ whose right-hand side contains a variable, replace every production $\alpha \to w$ with $w\in {\Sigma }^{\ast }$ by $\alpha \to w{S}^{\prime }$, include $P^{\prime }$, and use $S$ as start variable. If $G$ and $G^{\prime }$ are right-linear, the resulting grammar is right-linear and generates $L(G)L(G^{\prime })$.

The regular construction relies on a regular derivation having exactly one terminal rule, at its end. In a general variable-based grammar several variables may each use terminal productions, so replacing every such production can introduce several copies of the second start variable.