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I want to extract the outcode of a UK postcode. All the background is here: UK Postcode Regex (Comprehensive)

But it's not validation, so this should do:

  • first 2 characters for a postcode of length 5
  • first 3 characters for a postcode of length 6
  • first 4 characters for a postcode of length 7

All postcodes are converted to upper case and no spaces.

I cannot figure out how to specify dynamic ranges. Or whatever works.

Pseudo-code ^[A-Z0-9]{length(postcode) - 3}

Added: I'm using R.

Community
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Rico
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1 Answers1

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The language or environment (or rather the regex flavor) you're using would be helpful (always, in any regex question), but in most cases this should do:

^([A-Z0-9]{2,})[A-Z0-9]{3}$

So we match and capture 2 or more characters in group 1, and then require exactly 3 more until the end of the string. How you access the captures depends on your environment.

If your regex flavor supports lookaheads you get away without using captures as well:

^[A-Z0-9]{2,}(?=[A-Z0-9]{3}$)

This ensures that the end of the match is followed by three characters and the end of the string, but does not include this part in the match.

Martin Ender
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  • @Rico that is what my expressions do. The first one captures the outcode in group `1`, whereas the second returns only the outcode as the match. In R you might need to use PCRE regexes to use the latter. Have a look [here](http://stackoverflow.com/a/10038710/1633117) for how to get the first option working in R. – Martin Ender Jun 08 '13 at 16:14
  • Do you know how to extract group 1 in R from the first option? The second seems not work with the `stringr`package. – Rico Jun 08 '13 at 16:19
  • @Rico sorry I don't any R. But have a look at the link in my last comment. I suppose something along the lines of indexing the result with `[1,2]`? – Martin Ender Jun 08 '13 at 16:29
  • Yeah, I'm close but not quite there from an elegant solution. `rs[, (c("post1.len", "outcode")) := as.data.frame(str_match(lender.postcode, "^([A-Z0-9]{2,})[A-Z0-9]{3}$"), stringsAsFactors=FALSE)] ` Works. – Rico Jun 08 '13 at 16:32