Scala - replaceAllIn - regex

Question

First off, I'm new to Scala.
I'm trying to make a template parser in Scala (similar to Smarty (PHP)). It needs to search through the document, replacing anything inside "{{ }}" tags, with anything provided in the HashMap.
I'm currently stuck here:
import scala.collection.mutable.HashMap
import scala.io.Source
class Template(filename: String, vars: HashMap[Symbol, Any]) {
def parse() = {
var contents = Source.fromFile(filename, "ASCII").mkString
var rule = """\{\{(.*)\}\}""".r
//for(rule(v) <- rule findAllIn contents) {
// yield v
//}
//rule.replaceAllIn(contents, )
}
}
var t = new Template("FILENAME", new HashMap[Symbol, Any])
println(t.parse)
The part's that I've commented are things that I've thought about doing.
Thanks
I've come a little further...
import scala.collection.mutable.HashMap
import scala.io.Source
import java.util.regex.Pattern
import java.util.regex.Matcher
class Template(filename: String, vars: HashMap[Symbol, Any]) {
def findAndReplace(m: Matcher)(callback: String => String):String = {
val sb = new StringBuffer
while (m.find) {
m.appendReplacement(sb, callback(m.group(1)))
}
m.appendTail(sb)
sb.toString
}
def parse() = {
var contents = Source.fromFile(filename, "ASCII").mkString
val m = Pattern.compile("""\{\{(.*)\}\}""").matcher(contents)
findAndReplace(m){ x => x }
}
}
var t = new Template("FILENAME.html", new HashMap[Symbol, Any])
println(t.parse)
At the moment it just currently adds whatever was inside of the tag, back into the document. I'm wondering if there is an easier way of doing a find-and-replace style regexp in Scala?

Answer 1

I'd do it like this (String as key instead of Symbol):
var s : String = input // line, whatever
val regexp = """pattern""".r
while(regexp findFirstIn s != None) {
s = regexp replaceFirstIn (s, vars(regexp.findFirstIn(s).get))
}
If you prefer not using var, go recursive instead of using while. And, of course, a stringbuilder would be more efficient. In that case, I might do the following:
val regexp = """^(.*?)(?:{{(pattern)}})?""".r
for(subs <- regexp findAllIn s)
subs match {
case regexp(prefix, var) => sb.append(prefix); if (var != null) sb.append("{{"+vars(var)+"}}")
case _ => error("Shouldn't happen")
}
That way you keep appending the non-changing part, followed by the next part to be replaced.

Answer 2

There is a flavor of replaceAllIn in util.matching.Regex that accepts a replacer callback. A short example:
import util.matching.Regex
def replaceVars(r: Regex)(getVar: String => String) = {
def replacement(m: Regex.Match) = {
import java.util.regex.Matcher
require(m.groupCount == 1)
Matcher.quoteReplacement( getVar(m group 1) )
}
(s: String) => r.replaceAllIn(s, replacement _)
}
This is how we would use it:
val r = """\{\{([^{}]+)\}\}""".r
val m = Map("FILENAME" -> "aaa.txt",
"ENCODING" -> "UTF-8")
val template = replaceVars(r)( m.withDefaultValue("UNKNOWN") )
println( template("""whatever input contains {{FILENAME}} and
unknown key {{NOVAL}} and {{FILENAME}} again,
and {{ENCODING}}""") )
Note Matcher.quoteReplacement escapes $ characters in the replacement string. Otherwise you may get java.lang.IllegalArgumentException: Illegal group reference, replaceAll and dollar signs. See the blog post on why this may happen.

Answer 3

Here is also interesting way how to do the same using functions compose:
val Regexp = """\{\{([^{}]+)\}\}""".r
val map = Map("VARIABLE1" -> "VALUE1", "VARIABLE2" -> "VALUE2", "VARIABLE3" -> "VALUE3")
val incomingData = "I'm {{VARIABLE1}}. I'm {{VARIABLE2}}. And I'm {{VARIABLE3}}. And also {{VARIABLE1}}"
def replace(incoming: String) = {
def replace(what: String, `with`: String)(where: String) = where.replace(what, `with`)
val composedReplace = Regexp.findAllMatchIn(incoming).map { m => replace(m.matched, map(m.group(1)))(_) }.reduceLeftOption((lf, rf) => lf compose rf).getOrElse(identity[String](_))
composedReplace(incomingData)
}
println(replace(incomingData))
//OUTPUT: I'm VALUE1. I'm VALUE2. And I'm VALUE3. And also VALUE1