diff --git a/Cargo.lock b/Cargo.lock
index 0c30a92..8af5142 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -6,6 +6,15 @@ version = "1.0.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f26201604c87b1e01bd3d98f8d5d9a8fcbb815e8cedb41ffccbeb4bf593a35fe"
 
+[[package]]
+name = "aho-corasick"
+version = "0.7.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7404febffaa47dac81aa44dba71523c9d069b1bdc50a77db41195149e17f68e5"
+dependencies = [
+ "memchr",
+]
+
 [[package]]
 name = "ansi_term"
 version = "0.11.0"
@@ -938,6 +947,8 @@ version = "1.4.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "957056ecddbeba1b26965114e191d2e8589ce74db242b6ea25fc4062427a5c19"
 dependencies = [
+ "aho-corasick",
+ "memchr",
  "regex-syntax",
 ]
 
@@ -1278,8 +1289,10 @@ dependencies = [
  "clap",
  "cookie",
  "hmac-sha1",
+ "lazy_static 1.4.0",
  "log",
  "rand 0.7.3",
+ "regex",
  "reqwest",
  "rpassword",
  "rsa",
diff --git a/Cargo.toml b/Cargo.toml
index 1c92f00..3e2d098 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -20,4 +20,6 @@ standback = "0.2.17" # required to fix a compilation error on a transient depend
 clap = "2.33.3"
 log = "0.4.14"
 stderrlog = "0.4"
-cookie = "0.14"
\ No newline at end of file
+cookie = "0.14"
+regex = "1"
+lazy_static = "1.4.0"
diff --git a/src/main.rs b/src/main.rs
index 2bf7370..4db61a6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -7,9 +7,16 @@ use text_io::read;
 use std::{io::stdin, path::Path};
 use clap::{App, Arg, crate_version};
 use log::*;
+use regex::Regex;
 
+#[macro_use]
+extern crate lazy_static;
 mod accountmanager;
 
+lazy_static! {
+	static ref CAPTCHA_VALID_CHARS: Regex = Regex::new("^([A-H]|[J-N]|[P-R]|[T-Z]|[2-4]|[7-9]|[@%&])+$").unwrap();
+}
+
 fn main() {
 	let matches = App::new("steamguard-cli")
 		.version(crate_version!())
@@ -121,17 +128,7 @@ fn main() {
 						login.twofactor_code = account.generate_code(server_time);
 					}
 					steamapi::LoginResult::NeedCaptcha{ captcha_gid } => {
-						// example captchas:
-						// - 3982844815370620954
-						// - 3982844815370767244
-						// - 3982844815370804220
-						// - 3982844815370819607
-						println!("Captcha required. Open this link in your web browser: https://steamcommunity.com/public/captcha.php?gid={}", captcha_gid);
-						print!("Enter captcha text: ");
-						let _ = std::io::stdout().flush();
-						let mut captcha_text = String::new();
-						stdin().read_line(&mut captcha_text).expect("Did not enter a correct string");
-						login.captcha_text = String::from(captcha_text.strip_suffix('\n').unwrap());
+						login.captcha_text = prompt_captcha_text(&captcha_gid);
 					}
 					r => {
 						error!("Fatal login result: {:?}", r);
@@ -154,3 +151,47 @@ fn main() {
 		}
 	}
 }
+
+fn validate_captcha_text(text: &String) -> bool {
+	return CAPTCHA_VALID_CHARS.is_match(text);
+}
+
+#[test]
+fn test_validate_captcha_text() {
+	assert!(validate_captcha_text(&String::from("2WWUA@")));
+	assert!(validate_captcha_text(&String::from("3G8HT2")));
+	assert!(validate_captcha_text(&String::from("3J%@X3")));
+	assert!(validate_captcha_text(&String::from("2GCZ4A")));
+	assert!(validate_captcha_text(&String::from("3G8HT2")));
+	assert!(!validate_captcha_text(&String::from("asd823")));
+	assert!(!validate_captcha_text(&String::from("!PQ4RD")));
+	assert!(!validate_captcha_text(&String::from("1GQ4XZ")));
+	assert!(!validate_captcha_text(&String::from("8GO4XZ")));
+	assert!(!validate_captcha_text(&String::from("IPQ4RD")));
+	assert!(!validate_captcha_text(&String::from("0PT4RD")));
+	assert!(!validate_captcha_text(&String::from("APTSRD")));
+	assert!(!validate_captcha_text(&String::from("AP5TRD")));
+	assert!(!validate_captcha_text(&String::from("AP6TRD")));
+}
+
+/// Prompt the user for text input.
+fn prompt() -> String {
+	let mut text = String::new();
+	let _ = std::io::stdout().flush();
+	stdin().read_line(&mut text).expect("Did not enter a correct string");
+	return String::from(text.strip_suffix('\n').unwrap());
+}
+
+fn prompt_captcha_text(captcha_gid: &String) -> String {
+	println!("Captcha required. Open this link in your web browser: https://steamcommunity.com/public/captcha.php?gid={}", captcha_gid);
+	let mut captcha_text;
+	loop {
+		print!("Enter captcha text: ");
+		captcha_text = prompt();
+		if captcha_text.len() > 0 && validate_captcha_text(&captcha_text) {
+			break;
+		}
+		warn!("Invalid chars for captcha text found in user's input. Prompting again...");
+	}
+	return captcha_text;
+}