Sometimes it really matters when you forget to put ^ and $ in your RegExes.
Greek Letters in Greek Letters
eta in beta
eta in zeta
eta in theta
psi in epsilon
psi in upsilon
Elements in Elements
tin in platinum
tin in astatine
tin in actinium in protactinium
erbium in terbium in ytterbium
US States in US States
Kansas in Arkansas
Virginia in West Virginia
Countries in Countries
Cyprus in Northern Cyprus (disputed)
Mali in Somalia
Mali in Somaliland (disputed)
Niger in Nigeria
Oman in Romania
Sudan in South Sudan
Kerbies in Kerbies
[not a complete list, of course]
aabr in aabram in aabrams
aak in aaki in aaking
abala in abalan
abani in abanin
abar in abari
abass in abassman
abbi in babbin
abbi in heyrabbi
abass in seabass
aok in hexiaok
aok in xiaokhoo
ben in benazeer
ben in benkraft
dac in dactyl
dmass in dmasse
dove in lilydove
erf in maggierf
heyan in heyang
ich in ichuang
ike in mathmike
ike in mikemp
ike in shikexue
ike in shmike
jzen in jzeng
jzl in jzlee
jzy in jzyi
marc in emarcus
max in maxg
max in maxi in maxine
max in maxj
max in maxl
max in maxp
max in maxr
max in maxs in maxsh
max in maxv
max in maxwei
max in maxy
mer in gemery
mer in merolish
mer in wesommer
nes in mannes
raf in benkraft
rif in rifrim
seph in sepherke
sit in kasittig
tidor in btidor
tidor in jtidor
tron in astronut
tsims in ctsims
wes in wesommer
xchen in xcheng
xiaoyue in xiaoyuex
zwu in jzwu
zyu in zyuan
zyu in zyue
zyu in zyun
zzz in zzzdeng
zzz in zzzhang
MIT Classrooms in MIT Classrooms
1-390 in E51-390
Pokémon in Pokémon
Pigeot in Pigeotto
Paras in Parasect
Abra in Kadabra
Kabuto in Kabutops
Mew in Mewtwo
Marill in Azumarill
Porygon in Porygon2
Porygon in Porygon-Z
Woobat in Swoobat
Klink in Klinklang
Klang in Klinklang
Aron in Volcarona
LoL Champions in LoL Champions
Vi in Anivia
Vi in Sivir
Vi in Viktor