templates: add match function

Add a match function that returns true if a string matches the provided
regular expression. The compiled regular expressions are cached in
a sync.Map to avoid repetitive compilations of the same expressions.

Caveat: if the user mixes the arguments order, it may cause the cache to
consume a lot of memory filled with garbage regular expression objects.
Ideally, we would need to limit the size of this cache and/or use a LRU
cache implementation. Maybe someday?

Signed-off-by: Robin Jarry <robin@jarry.cc>
Acked-by: Tim Culverhouse <tim@timculverhouse.com>

3 files changed, 40 insertions, 0 deletions

diff --git a/doc/aerc-templates.7.scd b/doc/aerc-templates.7.scd
index 7d77cd0b..401c04e7 100644
--- a/doc/aerc-templates.7.scd
+++ b/doc/aerc-templates.7.scd
@@ -384,6 +384,14 @@ aerc provides the following additional functions:
 	X-Mailer: aerc {{version}}
 	```
 
+*match*
+	Check if a string matches a regular expression. This is intended for
+	use in conditional control flow:
+
+	```
+	{{if match .Subject `^(\[[\w-]+\]\s*)?\[(RFC )?PATCH`}}{{.Style .Subject "cyan"}}{{else}}{{.Subject}}{{end}}
+	```
+
 *Function chaining*
 	All of the template functions can be chained together if needed.
 
diff --git a/lib/parse/match.go b/lib/parse/match.go
new file mode 100644
index 00000000..ac43673e
--- /dev/null
+++ b/lib/parse/match.go
@@ -0,0 +1,30 @@
+package parse
+
+import (
+	"regexp"
+	"sync"
+
+	"git.sr.ht/~rjarry/aerc/log"
+)
+
+var reCache sync.Map
+
+// Check if a string matches the specified regular expression.
+// The regexp is compiled only once and stored in a cache for future use.
+func MatchCache(s, expr string) bool {
+	var re interface{}
+	var found bool
+
+	if re, found = reCache.Load(expr); !found {
+		var err error
+		re, err = regexp.Compile(expr)
+		if err != nil {
+			log.Errorf("`%s` invalid regexp: %s", expr, err)
+		}
+		reCache.Store(expr, re)
+	}
+	if re, ok := re.(*regexp.Regexp); ok && re != nil {
+		return re.MatchString(s)
+	}
+	return false
+}
diff --git a/lib/templates/functions.go b/lib/templates/functions.go
index 1cbdc85d..8bd8f37b 100644
--- a/lib/templates/functions.go
+++ b/lib/templates/functions.go
@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"git.sr.ht/~rjarry/aerc/lib/format"
+	"git.sr.ht/~rjarry/aerc/lib/parse"
 	"github.com/emersion/go-message/mail"
 )
 
@@ -271,4 +272,5 @@ var templateFuncs = template.FuncMap{
 	"join":          join,
 	"trimSignature": trimSignature,
 	"compactDir":    compactDir,
+	"match":         parse.MatchCache,
 }