10 REM REM Used the following as official standard:
20 REM REM  http://www.cnb.cz/cs/platebni_styk/iban/download/EBS204.pdf
30 REM REM Pairs of ISO 3166 country code & expected IBAN length for this country
40 LET c$="AL28 AD24 AT20 AZ28 BE16 BH22 BA20 BR29 BG22 CR21 HR21 CY28 CZ24 DK18 DO28 EE20 "
50 LET c$=c$+"FO18 FI18 FR27 GE22 DE22 GI23 GR27 GL18 GT28 HU28 IS26 IE22 IL23 IT27 KZ20 KW30 "
60 LET c$=c$+"LV21 LB28 LI21 LT20 LU20 MK19 MT31 MR27 MU30 MC27 MD24 ME22 NL18 NO15 PK24 PS29 "
70 LET c$=c$+"PL28 PT25 RO24 SM27 SA24 RS22 SK24 SI19 ES24 SE24 CH21 TN24 TR26 AE23 GB22 VG24 "
80 LET e$=""
100 LET i$="GB82 WEST 1234 5698 7654 32": GO SUB 1000
110 LET i$="gb82 west 1234 5698 7654 32": GO SUB 1000
120 LET i$="GB82 TEST 1234 5698 7654 32": GO SUB 1000
130 LET i$="GR16 0110 1250 0000 0001 2300 695": GO SUB 1000
140 LET i$="IL62-0108-0000-0009-9999-999": GO SUB 1000
900 STOP
1000 REM IBAN check routine
1010 LET explen=0: LET lenc=LEN c$
1020 FOR i=1 TO lenc STEP 5
1030 IF i$( TO 2)=c$(i TO i+1) THEN LET explen=VAL c$(i+2 TO i+3): GO TO 2000
1040 NEXT i
2000 LET match=explen>0
2010 REM Continue if country code found
2020 IF NOT match THEN LET e$="country code: "+i$( TO 2): GO TO 3000
2030 REM Remove espace = convert to digital IBAN
2040 LET d$=""
2050 FOR i=1 TO LEN i$
2060 IF i$(i)>" " THEN LET d$=d$+i$(i)
2070 NEXT i
2080 LET match=explen=LEN d$
2090 REM Continue if length is correct
2100 IF NOT match THEN LET e$="code length, expected length: "+STR$ explen: GO TO 3000
2110 REM Create temporary string with country code appended
2120 LET t$=d$(5 TO )+d$(1 TO 2)
2130 REM Make big number, replacing letters by numbers using next conversion table: A=10 ... Z=35
2140 LET b$=""
2150 FOR i=1 TO LEN t$
2160 LET c= CODE t$(i)
2170 IF (c>64 AND c<91) THEN LET b$=b$+STR$ (c-55): GO TO 2200
2190 IF (c>47 AND c<58) THEN LET b$=b$+t$(i)
2200 NEXT i
2210 REM MOD 97 on bignum$+"00" and subtract result from 98 to obtain control number
2220 LET k$=b$+"00": GO SUB 4000
2230 LET kk=98-mod97
2240 REM Compare with control number in IBAN
2250 LET match=VAL i$(3 TO 4)=kk
2260 REM Continue if control number matches
2270 IF NOT match THEN LET e$="check digits, should be: "+STR$ kk: GO TO 3000
2280 REM Append kk to bignum$ and determine if MOD 97 results in 1
2285 LET k$="0"+STR$ kk
2290 LET k$=b$+k$(LEN k$-1 TO ): GO SUB 4000
2300 LET match=mod97=1
2310 REM Continue if mod 97
2320 IF NOT match THEN LET e$="result from modulo 97"
3000 IF match THEN PRINT "  ": GO TO 3100
3010 PRINT "in";: LET e$=" ***error!*** invalid "+e$
3100 PRINT "valid IBAN: ";i$;e$: LET e$=""
3110 RETURN
4000 REM Modulo 97
4010 LET mod97=0
4020 FOR i=1 TO LEN k$
4030 LET d$=STR$ (mod97)+k$(i)
4040 LET mod97=FN m(VAL (d$),97)
4050 NEXT i
4060 RETURN
5000 DEF FN m(a,b)=a-INT (a/b)*b: REM modulo