Assuming in Cell G2 the following data:
AMEX:ARY, AMEX:BDM, AMEX:BHM, AMEX:BPN, AMEX:BWR, AMEX:BXA, AMEX:BXU,
AMEX:CSM, AMEX:DCV, AMEX:DMD, AMEX:DMP, AMEX:DSA, AMEX:DSE, AMEX:DSI,
AMEX:DSJ, AMEX:DSK, AMEX:DSN, AMEX:DSO, AMEX:DSP, AMEX:DSZ, AMEX:DTY,
AMEX:EST, AMEX:ESY, AMEX:EUM, AMEX:GWM, AMEX:IHM, AMEX:LDB, AMEX:MCP,
AMEX:MDJ, AMEX:MHR, AMEX:MIS, AMEX:MKO, AMEX:MKP, AMEX:MLJ, AMEX:MLN,
AMEX:MLW, AMEX:MNK, AMEX:MNM, AMEX:MPF, AMEX:MPL, AMEX:MTT, AMEX:NKB,
AMEX:NKM, AMEX:NKS, AMEX:NKW, AMEX:NML, AMEX:OGN, AMEX:PPE, AMEX:RNI,
AMEX:RRM, AMEX:RSM, AMEX:SME, AMEX:SRK, AMEX:UUD, AMEX:UUY, BASE:MER,
BMV:MER, DB:MER, NasdaqNM:ARGB, NasdaqNM:ARQQ, NasdaqNM:ARRB, NasdaqNM:CSJB,
NasdaqNM:DOTN, NasdaqNM:DOWT, NasdaqNM:DWID, NasdaqNM:DWMT, NasdaqNM:DWTN,
NasdaqNM:DWTT, NasdaqNM:LERA, NasdaqNM:LNDU, NasdaqNM:MITT, NasdaqNM:MLMT,
NasdaqNM:MNNY, NasdaqNM:MSPX, NasdaqNM:MTDB, NasdaqNM:MTDW, NasdaqNM:MTNK,
NasdaqNM:MTSM, NasdaqNM:MTSP, NasdaqNM:MTTT, NasdaqNM:MTTX, NasdaqNM:PDNT,
NasdaqNM:PGEB, NasdaqNM:SPPX, NasdaqNM:SRDD, NasdaqNM:SRIX, NasdaqNM:SRRR,
NYSE:IEM, NYSE:MER.PRG, NYSE:MER.PRH, NYSE:MER.PRI, NYSE:MER.PRJ,
OTCPK:MERI.Z, OTCPK:MEXZ, NYSE:MER

I just want to return in Cell H2: "NYSE:MER"

That means the formula will:
1) look at text strings that follow "NYSE:" and precede "." and return only
that text that matches that exact criteria.
2) This formula is an extension of the "BigNum" solution that Domenic listed.

Thanks very much!