message_regexp: regular expression rule for messages

Fixes: #30

now you can add a regular expression to the rules to be run
i.e. `git-validation -run "dco,message_regexp='^JIRA-[0-9]+ [A-Z].*$'"`

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

rules/messageregexp/rule.go

@@ -0,0 +1,61 @@
+package messageregexp
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+
+	"github.com/vbatts/git-validation/git"
+	"github.com/vbatts/git-validation/validate"
+)
+
+func init() {
+	validate.RegisterRule(RegexpRule)
+}
+
+var (
+	// RegexpRule for validating a user provided regex on the commit messages
+	RegexpRule = validate.Rule{
+		Name:        "message_regexp",
+		Description: "checks the commit message for a user provided regular expression",
+		Run:         ValidateMessageRegexp,
+		Default:     false, // only for users specifically calling it through -run ...
+	}
+)
+
+// ValidateMessageRegexp is the message regex func to run
+func ValidateMessageRegexp(r validate.Rule, c git.CommitEntry) (vr validate.Result) {
+	if r.Value == "" {
+		vr.Pass = true
+		vr.Msg = "noop: message_regexp value is blank"
+		return vr
+	}
+
+	re := regexp.MustCompile(r.Value)
+	vr.CommitEntry = c
+	if len(strings.Split(c["parent"], " ")) > 1 {
+		vr.Pass = true
+		vr.Msg = "merge commits are not checked for message_regexp"
+		return vr
+	}
+
+	hasValid := false
+	for _, line := range strings.Split(c["subject"], "\n") {
+		if re.MatchString(line) {
+			hasValid = true
+		}
+	}
+	for _, line := range strings.Split(c["body"], "\n") {
+		if re.MatchString(line) {
+			hasValid = true
+		}
+	}
+	if !hasValid {
+		vr.Pass = false
+		vr.Msg = fmt.Sprintf("commit message does not match %q", r.Value)
+	} else {
+		vr.Pass = true
+		vr.Msg = fmt.Sprintf("commit message matches %q", r.Value)
+	}
+	return vr
+}