Anchor regex meta-rules.

README.md

@@ -58,6 +58,8 @@ improvements.
      `-p=1` if this is the case.
   1. Use Go regular expressions, which are perl-like. The original mk used plan9
      regex, which few people know or care to learn.
+  1. Regex matches are substituted into rule prerequisites with `$stem1`,
+     `$stem2`, etc, rather than `\1`, `\2`, etc.
   1. Allow blank lines in recipes. A recipe is any indented block of text, and
      continues until a non-indented character or the end of the file. (Similar
      to blocks in Python.)

parse.go

@@ -308,7 +308,7 @@ func parseRecipe(p *parser, t token) parserStateFun {
 			r.targets = append(r.targets, pattern{spat: targetstr})
 
 			if r.attributes.regex {
-				rpat, err := regexp.Compile(targetstr)
+				rpat, err := regexp.Compile("^" + targetstr + "$")
 				if err != nil {
 					msg := fmt.Sprintf("invalid regular expression: %q", err)
 					p.basicErrorAtToken(msg, p.tokenbuf[k])