The group that matches the postcode area can be grouped, reducing the overall regex length:
Original
Whole regex: GIR ?0AA|(?:(?:AB|AL|B|BA|BB|BD|BF|BH|BL|BN|BR|BS|BT|BX|CA|CB|CF|CH|CM|CO|CR|CT|CV|CW|DA|DD|DE|DG|DH|DL|DN|DT|DY|E|EC|EH|EN|EX|FK|FY|G|GL|GY|GU|HA|HD|HG|HP|HR|HS|HU|HX|IG|IM|IP|IV|JE|KA|KT|KW|KY|L|LA|LD|LE|LL|LN|LS|LU|M|ME|MK|ML|N|NE|NG|NN|NP|NR|NW|OL|OX|PA|PE|PH|PL|PO|PR|RG|RH|RM|S|SA|SE|SG|SK|SL|SM|SN|SO|SP|SR|SS|ST|SW|SY|TA|TD|TF|TN|TQ|TR|TS|TW|UB|W|WA|WC|WD|WF|WN|WR|WS|WV|YO|ZE)(?:\d[\dA-Z]? ?\d[ABD-HJLN-UW-Z]{2}))|BFPO ?\d{1,4}
Area group: (?:(?:AB|AL|B|BA|BB|BD|BF|BH|BL|BN|BR|BS|BT|BX|CA|CB|CF|CH|CM|CO|CR|CT|CV|CW|DA|DD|DE|DG|DH|DL|DN|DT|DY|E|EC|EH|EN|EX|FK|FY|G|GL|GY|GU|HA|HD|HG|HP|HR|HS|HU|HX|IG|IM|IP|IV|JE|KA|KT|KW|KY|L|LA|LD|LE|LL|LN|LS|LU|M|ME|MK|ML|N|NE|NG|NN|NP|NR|NW|OL|OX|PA|PE|PH|PL|PO|PR|RG|RH|RM|S|SA|SE|SG|SK|SL|SM|SN|SO|SP|SR|SS|ST|SW|SY|TA|TD|TF|TN|TQ|TR|TS|TW|UB|W|WA|WC|WD|WF|WN|WR|WS|WV|YO|ZE)
Modified regex
Whole regex: GIR ?0AA|(?:(?:A[BL]|B[ABDFHLNRSTX]?|C[ABFHMORTVW]|D[ADEGHLNTY]|E[CHNX]?|F[KY]|G[LYU]?|H[ADGPRSUX]|I[GMPV]|JE|K[ATWY]|L[ADELNSU]?|M[EKL]?|N[EGNPRW]?|O[LX]|P[AEHLOR]|R[GHM]|S[AEGK-PRSTWY]?|T[ADFNQRSW]|UB|W[ACDFNRSV]?|YO|ZE)(?:\d[\dA-Z]? ?\d[ABD-HJLN-UW-Z]{2}))|BFPO ?\d{1,4}
Area group: (?:A[BL]|B[ABDFHLNRSTX]?|C[ABFHMORTVW]|D[ADEGHLNTY]|E[CHNX]?|F[KY]|G[LYU]?|H[ADGPRSUX]|I[GMPV]|JE|K[ATWY]|L[ADELNSU]?|M[EKL]?|N[EGNPRW]?|O[LX]|P[AEHLOR]|R[GHM]|S[AEGK-PRSTWY]?|T[ADFNQRSW]|UB|W[ACDFNRSV]?|YO|ZE)
The group that matches the postcode area can be grouped, reducing the overall regex length:
Original
Whole regex:
GIR ?0AA|(?:(?:AB|AL|B|BA|BB|BD|BF|BH|BL|BN|BR|BS|BT|BX|CA|CB|CF|CH|CM|CO|CR|CT|CV|CW|DA|DD|DE|DG|DH|DL|DN|DT|DY|E|EC|EH|EN|EX|FK|FY|G|GL|GY|GU|HA|HD|HG|HP|HR|HS|HU|HX|IG|IM|IP|IV|JE|KA|KT|KW|KY|L|LA|LD|LE|LL|LN|LS|LU|M|ME|MK|ML|N|NE|NG|NN|NP|NR|NW|OL|OX|PA|PE|PH|PL|PO|PR|RG|RH|RM|S|SA|SE|SG|SK|SL|SM|SN|SO|SP|SR|SS|ST|SW|SY|TA|TD|TF|TN|TQ|TR|TS|TW|UB|W|WA|WC|WD|WF|WN|WR|WS|WV|YO|ZE)(?:\d[\dA-Z]? ?\d[ABD-HJLN-UW-Z]{2}))|BFPO ?\d{1,4}Area group:
(?:(?:AB|AL|B|BA|BB|BD|BF|BH|BL|BN|BR|BS|BT|BX|CA|CB|CF|CH|CM|CO|CR|CT|CV|CW|DA|DD|DE|DG|DH|DL|DN|DT|DY|E|EC|EH|EN|EX|FK|FY|G|GL|GY|GU|HA|HD|HG|HP|HR|HS|HU|HX|IG|IM|IP|IV|JE|KA|KT|KW|KY|L|LA|LD|LE|LL|LN|LS|LU|M|ME|MK|ML|N|NE|NG|NN|NP|NR|NW|OL|OX|PA|PE|PH|PL|PO|PR|RG|RH|RM|S|SA|SE|SG|SK|SL|SM|SN|SO|SP|SR|SS|ST|SW|SY|TA|TD|TF|TN|TQ|TR|TS|TW|UB|W|WA|WC|WD|WF|WN|WR|WS|WV|YO|ZE)Modified regex
Whole regex:
GIR ?0AA|(?:(?:A[BL]|B[ABDFHLNRSTX]?|C[ABFHMORTVW]|D[ADEGHLNTY]|E[CHNX]?|F[KY]|G[LYU]?|H[ADGPRSUX]|I[GMPV]|JE|K[ATWY]|L[ADELNSU]?|M[EKL]?|N[EGNPRW]?|O[LX]|P[AEHLOR]|R[GHM]|S[AEGK-PRSTWY]?|T[ADFNQRSW]|UB|W[ACDFNRSV]?|YO|ZE)(?:\d[\dA-Z]? ?\d[ABD-HJLN-UW-Z]{2}))|BFPO ?\d{1,4}Area group:
(?:A[BL]|B[ABDFHLNRSTX]?|C[ABFHMORTVW]|D[ADEGHLNTY]|E[CHNX]?|F[KY]|G[LYU]?|H[ADGPRSUX]|I[GMPV]|JE|K[ATWY]|L[ADELNSU]?|M[EKL]?|N[EGNPRW]?|O[LX]|P[AEHLOR]|R[GHM]|S[AEGK-PRSTWY]?|T[ADFNQRSW]|UB|W[ACDFNRSV]?|YO|ZE)