Nucleophilic attack on cyclic intermediates in dibromination and oxymercuration reactions of alkenes
Just to clear your doubts these are electrophilic addition reactions

Case 1: oxymercuation-demercuration

The reaction is initiated by electrophilic addition of $\ce{Hg^2+}$(mercuric) ion to the alkene to give an intermediate mercurinium ion, whose structure resembles that of a bromonium ion. Nucleophilic addition of water as in halohydrin formation, followed by loss of a proton, then yields a stable organomercury product.
The final step, demercuration of the organomercury compound by reaction with sodium borohydride, is complex and involves radicals.

The regiochemistry of the reaction corresponds to Markovnikov
addition of water; that is, the $\ce{-OH}$ group attaches to the more highly substituted carbon atom, and the  $\ce{-H}$ attaches to the less highly substituted carbon.


  When the π electrons of the alkene attack the mercury cation, a
  three-membered intermediate forms in which both carbons share a single
  bond with the mercury and a partial positive charge. It turns out that
  rearrangements are unlikely to occur from this intermediate (as
  opposed to the discreet carbocation intermediate in the acidic
  hydration reaction).


A water molecule then attacks the more substituted carbon, which bears a greater share of the positive charge. Cleavage of the carbon-Hg bond is accomplished with a reducing agent called sodium borohydride $\ce{NaBH4}$.

See images:





So where is the carbocation present?



The end result of an oxymercuration-reduction reaction is the Markovnikov addition of water to an alkene without rearrangement

The regioselectivity on this reaction is partly based on inductive effects resulting from the shifting of electrons in a s bond in response to the electronegativity of nearby atoms. I believe the other reason is based on hydride shift which converts the carbonation into a tertiary one and this is thermodynamically favourable. (Not entirely sure on second reason though)

Case 2: Dibromination

Halogenation of Alkenes: Addition of $\ce{Br2}$

Follows the reaction sequence (proposed by George Kimball and Irving Roberts) 


  that the reaction intermediate is not a carbocation but is instead a
  bromonium ion, $\ce{R2Br+}$, formed by electrophilic addition of $\ce{Br+}$ to the
  alkene.


The bromonium ion is formed in a single step by interaction of the alkene with $\ce{Br2}$ and simultaneous
loss of $\ce{Br-}$.

See image:



I have updated to include an even more detailed mechanism: