diff --git a/validator/Cargo.toml b/validator/Cargo.toml
index 212cf1e6..bb341f17 100644
--- a/validator/Cargo.toml
+++ b/validator/Cargo.toml
@@ -12,7 +12,7 @@ readme = "../README.md"
 
 [dependencies]
 url = "2"
-regex = "1"
+regex = { version = "1", default-features = false, features = ["std"] }
 lazy_static = "1"
 idna = "0.2"
 serde = "1"
diff --git a/validator/src/validation/email.rs b/validator/src/validation/email.rs
index 0b386eba..8250013c 100644
--- a/validator/src/validation/email.rs
+++ b/validator/src/validation/email.rs
@@ -9,12 +9,12 @@ lazy_static! {
     // Regex from the specs
     // https://html.spec.whatwg.org/multipage/forms.html#valid-e-mail-address
     // It will mark esoteric email addresses like quoted string as invalid
-    static ref EMAIL_USER_RE: Regex = Regex::new(r"^(?i)[a-z0-9.!#$%&'*+/=?^_`{|}~-]+\z").unwrap();
+    static ref EMAIL_USER_RE: Regex = Regex::new(r"^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+\z").unwrap();
     static ref EMAIL_DOMAIN_RE: Regex = Regex::new(
-        r"(?i)^[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?(?:\.[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?)*$"
+        r"^[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$"
     ).unwrap();
     // literal form, ipv4 or ipv6 address (SMTP 4.1.3)
-    static ref EMAIL_LITERAL_RE: Regex = Regex::new(r"(?i)\[([A-f0-9:\.]+)\]\z").unwrap();
+    static ref EMAIL_LITERAL_RE: Regex = Regex::new(r"\[([a-fA-F0-9:\.]+)\]\z").unwrap();
 }
 
 /// Validates whether the given string is an email based on the [HTML5 spec](https://html.spec.whatwg.org/multipage/forms.html#valid-e-mail-address).
diff --git a/validator_derive/Cargo.toml b/validator_derive/Cargo.toml
index a7d2201c..8e784642 100644
--- a/validator_derive/Cargo.toml
+++ b/validator_derive/Cargo.toml
@@ -24,7 +24,7 @@ proc-macro2 = "1"
 proc-macro-error = "1"
 if_chain = "1"
 validator_types = { version = "0.16", path = "../validator_types" }
-regex = "1.5.5"
+regex = { version = "1.5.5", default-features = false, features = ["std"] }
 lazy_static = "1"