fix(normalizer): strip each country's own trunk prefix once

Readme.md

@@ -138,6 +138,23 @@ Sanja includes comprehensive country data for **250+ countries and territories**
 ### Data Source
 The country data used in this package was sourced from [Kaggle - Country 2ISO3UN Digit Code and Dialing Code](https://www.kaggle.com/datasets/migeruj/country-2iso3un-digit-code-and-dialing-code). The main country for each shared dialling code (`mainCountryForCode`) and the prefix each country dials to call abroad (`internationalPrefix`) come from Google’s [libphonenumber metadata](https://github.com/google/libphonenumber/blob/master/resources/PhoneNumberMetadata.xml).
 
+### Updating country data
+
+These fields in `countries.json` come from Google’s libphonenumber metadata:
+
+- `internationalPrefix`: what callers dial before a foreign number, such as `00` or `011`
+- `nationalPrefix`: what callers dial before a number inside the country, such as `0` in Malawi or `8` in Russia. Some countries, like Italy and Côte d’Ivoire, have none: their leading `0` is part of the number
+- `numberPattern`: the shape of a valid number, used to tell a leading `0` or `8` that belongs to the number from one that doesn’t
+- `mainCountryForCode`: which country to use when several share a dialling code
+
+To refresh them, pin the libphonenumber release in `internal/cmd/gencountries/main.go` and run this from the repository root:
+
+```bash
+go run ./internal/cmd/gencountries
+```
+
+Countries libphonenumber doesn’t list keep their current values, and the command prints their codes.
+
 ## API Reference
 
 ### Types
@@ -150,6 +167,8 @@ type Country struct {
     NumCode             int    // ISO Numeric code (e.g., 840)
     DialingCode         string // International dialing code (e.g., "1"), or several separated by commas
     InternationalPrefix string // Pattern for what callers dial before a foreign number (e.g., "00", "011")
+    NationalPrefix      string // What callers dial before a number inside the country (e.g., "0"), or "" for none
+    NumberPattern       string // Pattern for a valid number without the dialing code
     MainCountryForCode  bool   // True for the country returned when several share a dialing code
     MinLocalDigits      int    // Fewest digits after the dialing code
     MaxLocalDigits      int    // Most digits after the dialing code

countries.go

@@ -24,6 +24,13 @@ func parseCountries() []Country {
 		}
 
 		result[i].internationalPrefixPattern = pattern
+
+		numberPattern, err := compileNumberPattern(result[i].NumberPattern)
+		if err != nil {
+			panic("sanja: number pattern for " + result[i].A2 + ": " + err.Error())
+		}
+
+		result[i].numberPattern = numberPattern
 	}
 
 	return result