Tiny awk web template

templ.awk

function die (msg)
{
    printf "%s:%d %s\n", FILENAME, FNR, msg > "/dev/stderr"
    exit 1
}

BEGIN { FS="{!}"; OFS="" }
NF > 0 && NF % 2 == 0 { die("Unbalanced {!} tokens.") }
NF > 2 {
    for (i=2; i<=NF; i+=2) {
        if (!match($i, "^[A-Za-z]+$")) {
            die("Invalid variable name: \"" $i "\"")
        }
        if (ENVIRON[$i] == "") {
            die("Undefined variable: \"" $i "\"")
        }
        $i = ENVIRON[$i]
    }
}
{ print }