diff --git a/src/atom/parsers.rs b/src/atom/parsers.rs
index 65cce04..f4686f2 100644
--- a/src/atom/parsers.rs
+++ b/src/atom/parsers.rs
@@ -280,4 +280,11 @@ mod test {
 
         atom().check_finished(it).unwrap();
     }
+
+    #[test]
+    fn test_usedep_with_underscore() {
+        let it = InputIter::new("foo/bar-1.0.0[use_dep]");
+
+        atom().check_finished(it).unwrap();
+    }
 }
diff --git a/src/useflag/parsers.rs b/src/useflag/parsers.rs
index e6e228b..a430b3d 100644
--- a/src/useflag/parsers.rs
+++ b/src/useflag/parsers.rs
@@ -1,10 +1,15 @@
-use mon::{Parser, alpha1, alphanumeric, one_of, take_while};
+use mon::{Parser, r#if, take_while};
 
 use crate::useflag::UseFlag;
 
 pub fn useflag<'a>() -> impl Parser<&'a str, Output = UseFlag> {
-    alpha1()
-        .and(alphanumeric().or(take_while(one_of("+_@-".chars()))))
+    let start = r#if(|c: &char| c.is_ascii_alphanumeric());
+    let rest = take_while(r#if(|c: &char| {
+        c.is_ascii_alphanumeric() || "+_@-".contains(*c)
+    }));
+
+    start
+        .and(rest)
         .recognize()
         .map(|output: &str| UseFlag(output.to_string()))
 }