next.js/packages/next/src/server/app-render/get-segment-param.tsx

import { INTERCEPTION_ROUTE_MARKERS } from '../../shared/lib/router/utils/interception-routes'
import type { DynamicParamTypes } from './types'

/**
 * Parse dynamic route segment to type of parameter
 */
export function getSegmentParam(segment: string): {
  param: string
  type: DynamicParamTypes
} | null {
  const interceptionMarker = INTERCEPTION_ROUTE_MARKERS.find((marker) =>
    segment.startsWith(marker)
  )

  // if an interception marker is part of the path segment, we need to jump ahead
  // to the relevant portion for param parsing
  if (interceptionMarker) {
    segment = segment.slice(interceptionMarker.length)
  }

  if (segment.startsWith('[[...') && segment.endsWith(']]')) {
    return {
      // TODO-APP: Optional catchall does not currently work with parallel routes,
      // so for now aren't handling a potential interception marker.
      type: 'optional-catchall',
      param: segment.slice(5, -2),
    }
  }

  if (segment.startsWith('[...') && segment.endsWith(']')) {
    return {
      type: interceptionMarker ? 'catchall-intercepted' : 'catchall',
      param: segment.slice(4, -1),
    }
  }

  if (segment.startsWith('[') && segment.endsWith(']')) {
    return {
      type: interceptionMarker ? 'dynamic-intercepted' : 'dynamic',
      param: segment.slice(1, -1),
    }
  }

  return null
}