Regular black holes, free of central singularities, provide an ideal laboratory for probing the geometric structure of spacetime. The global structure of some regular black holes, e.g., Hayward black hole, features an event horizon and a Cauchy horizon, raising fundamental questions about the latter’s stability. In this work, we investigate the collapse of a scalar field in Hayward spacetime. Under weak scalar perturbations, the inner horizon maintains a stable finite radius. In the circumstance of a strong scalar field, the inner horizon shrinks to zero volume, accompanied by the formation of a spacelike singularity. The Hayward geometry is effectively converted into a Schwarzschild-like geometry. Furthermore, the characteristic parameter of the scalar field governs the contraction dynamics of the inner horizon. As the parameter $p$ of the initial profile for the scalar field approaches the critical threshold $p_{*}$, the radius of the inner horizon $r_{-}$ exhibits a universal scaling behavior: $r_{-}\propto |p-p_{*}{|}^{\gamma }$, with a critical exponent $\gamma \approx 0.5$.

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• Classical black holes
• General relativity
• Singularities In general relativity
• Numerical relativity
• Numerical simulations in gravitation & astrophysics