Re: st: problem with regexm leading to "regexp: unterminated ()" error for all observations

[Phil Schumm <pschumm@uchicago.edu>]

On Jun 3, 2011, at 7:35 AM, Jamie Fagg wrote:
> I've a problem with the function -regexm-. I get the following  
> message:
>
> regexp: unterminated ()

<snip>

> #delimit ;
>
> //regular expression to define whether postcode is syntactically  
> correct
>
> ge postcodevalid = 1 if regexm(postcode,"(GIR 0AA)|(((A[BL]| 
> B[ABDHLNRSTX]
> ?|C[ABFHMORTVW]|D[ADEGHLNTY]|E[HNX]?|F[KY]|G[LUY]?|H[ADGPRSUX]
> |I[GMPV]|JE|K[ATWY]|L[ADELNSU]?|M[EKL]?|N[EGNPRW]?|O[LX]|P[AEHLOR]
> |R[GHM]|S[AEGKLMNOPRSTY]?|T[ADFNQRSW]|UB|W[ADFNRSV]|YO|ZE)[1-9]?[0-9]
> |((E|N|NW|SE|SW|W)1|EC[1-4]|WC[12])[A-HJKMNPR-Y]|(SW|W)([2-9]|[1-9]
> [0-9])|EC[1-9][0-9]) [0-9][ABD-HJLNP-UW-Z]{2})")==1;


I'm not sure why Stata chokes on this, though I would suspect it might  
have something to do with the length.  As Nick and Steven have already  
noted, the repeat qualifier {n} is not supported by Stata's regular  
expression syntax, so you'll need to replace


    [ABD-HJLNP-UW-Z]{2}


with the equivalent


    [ABD-HJLNP-UW-Z][ABD-HJLNP-UW-Z]


Now, Nick suggested breaking the expression up, so let's do that.   
Your expression is equal to


    (p1)|(((p2a1a|p2a1b|p2a1c)p2a1d|p2a2|p2a3|p2a4)p2b)


where the individual parts (as assigned to Stata macros) are


    loc p1    "GIR 0AA"
    loc p2a1a "A[BL]|B[ABDHLNRSTX]?|C[ABFHMORTVW]|D[ADEGHLNTY]| 
E[HNX]?|F[KY]|G[LUY]?"
    loc p2a1b "H[ADGPRSUX]|I[GMPV]|JE|K[ATWY]|L[ADELNSU]?|M[EKL]?| 
N[EGNPRW]?|O[LX]"
    loc p2a1c "P[AEHLOR]|R[GHM]|S[AEGKLMNOPRSTY]?|T[ADFNQRSW]|UB| 
W[ADFNRSV]|YO|ZE"
    loc p2a1d "[1-9]?[0-9]"
    loc p2a2  "((E|N|NW|SE|SW|W)1|EC[1-4]|WC[12])[A-HJKMNPR-Y]"
    loc p2a3  "(SW|W)([2-9]|[1-9][0-9])"
    loc p2a4  "EC[1-9][0-9]"
    loc p2b   " [0-9][ABD-HJLNP-UW-Z][ABD-HJLNP-UW-Z]"


This may then be easily broken up as follows:


    gen byte valid = regexm(postcode,"`p1'")
    replace valid = 1 if regexm(postcode,"`p2a1a'`p2a1d'`p2b'")
    replace valid = 1 if regexm(postcode,"`p2a1b'`p2a1d'`p2b'")
    replace valid = 1 if regexm(postcode,"`p2a1c'`p2a1d'`p2b'")
    replace valid = 1 if regexm(postcode,"(`p2a2'|`p2a3'|`p2a4')`p2b'")


-- Phil

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