custom template functions with regex https://play.golang.org/p/NSA9sWyvdAd

template-custom-func-regexp.go

package main

import (
	"log"
	"os"
	"regexp"
	"strings"
	"text/template"
)

// regexFindAllSubmatch returns all the matching groups
// [0] string matches the whole regex
// [1:] strings matches the n-th group
func regexFindAllSubmatch(regex string, s string) []string {
	r := regexp.MustCompile(regex)
	return r.FindStringSubmatch(s)
}

func main() {
	// First we create a FuncMap with which to register the function.
	funcMap := template.FuncMap{
		// The name "title" is what the function will be called in the template text.
		"title": strings.Title,
		"re1":   regexFindAllSubmatch,
	}

	// A simple template definition to test our function.
	// We print the input text several ways:
	// - the original
	// - title-cased
	// - title-cased and then printed with %q
	// - printed with %q and then title-cased.
	const templateText = `
Input: {{printf "%q" .}}
Output 0: {{title .}}
Output 1: {{title . | printf "%q"}}
Output 2: {{printf "%q" . | title}}
Output 3: {{ "peach" | re1 "p([a-z]+)ch" }}
Output 4: {{ . | re1 "([a-z]+)a(mm[a-z]+)" }}
`

	// Create a template, add the function map, and parse the text.
	tmpl, err := template.New("titleTest").Funcs(funcMap).Parse(templateText)
	if err != nil {
		log.Fatalf("parsing: %s", err)
	}

	// Run the template to verify the output.
	err = tmpl.Execute(os.Stdout, "the go programming language")
	if err != nil {
		log.Fatalf("execution: %s", err)
	}

}