diff --git a/questdb-confstr/README.md b/questdb-confstr/README.md
index 2a89f85..6bdffd3 100644
--- a/questdb-confstr/README.md
+++ b/questdb-confstr/README.md
@@ -14,7 +14,7 @@ service::key1=value1;key2=value2;key3=value3;
 A few rules:
 * The last semicolon is mandatory.
 * Service name and keys are case-insensitive.
-* Keys are ASCII alphanumeric and start with a letter.
+* Keys are ASCII alphanumeric and can contain underscores.
 * Values are case-sensitive unicode strings which can contain any characters,
   * Except control characters (`0x00..=0x1f` and `0x7f..=0x9f`).
   * If semicolons `;` appears in a value, these are escaped as double semicolon `;;`.
@@ -29,9 +29,8 @@ param ::= key "=" value
 key ::= identifier
 value ::= { value_char }
 
-identifier ::= alpha { alphanumeric }
-alpha ::= "a".."z" | "A".."Z"
-alphanumeric ::= "a".."z" | "A".."Z" | "0".."9"
+identifier ::= alpha_num_under { alpha_num_under }
+alpha_num_under ::= "a".."z" | "A".."Z" | "0".."9" | "_"
 value_char ::= non_semicolon_char | escaped_semicolon
 escaped_semicolon ::= ";;"
 non_semicolon_char ::= ? any unicode character except ';', 0x00..=0x1f and 0x7f..=0x9f ?
diff --git a/questdb-confstr/src/lib.rs b/questdb-confstr/src/lib.rs
index c7246e6..b0c20e6 100644
--- a/questdb-confstr/src/lib.rs
+++ b/questdb-confstr/src/lib.rs
@@ -174,12 +174,10 @@ fn parse_ident(
     let mut token = String::new();
     while let Some((pos, c)) = iter.peek0() {
         *next_pos = *pos;
-        let take = if token.is_empty() {
-            c.is_ascii_alphabetic()
+        if c.is_ascii_alphanumeric() || *c == '_' {
+            token.push(c.to_ascii_lowercase());
+            iter.next();
         } else {
-            c.is_ascii_alphanumeric()
-        };
-        if !take {
             if token.is_empty() {
                 return Err(parse_err(ErrorKind::ExpectedIdentifierNot(*c), *next_pos));
             } else if !c.is_ascii() || matches!(c, '\0'..=' ') {
@@ -187,8 +185,6 @@ fn parse_ident(
             }
             break;
         }
-        token.push(c.to_ascii_lowercase());
-        iter.next();
     }
 
     if token.is_empty() {
diff --git a/questdb-confstr/tests/tests.rs b/questdb-confstr/tests/tests.rs
index fb295b7..c304d9f 100644
--- a/questdb-confstr/tests/tests.rs
+++ b/questdb-confstr/tests/tests.rs
@@ -22,6 +22,7 @@
  *
  ******************************************************************************/
 
+use std::collections::HashMap;
 use questdb_confstr::{parse_conf_str, ErrorKind, ParsingError};
 
 #[test]
@@ -75,17 +76,24 @@ fn duplicate_key() {
 }
 
 #[test]
-fn key_must_start_with_letter() {
+fn key_can_start_with_number() -> Result<(), ParsingError> {
     let input = "https::123=456;";
-    let config = parse_conf_str(input);
-    assert!(config.is_err());
-    let err = config.unwrap_err();
-    assert_eq!(err.kind().clone(), ErrorKind::ExpectedIdentifierNot('1'));
-    assert_eq!(err.position(), 7);
-    assert_eq!(
-        err.to_string(),
-        "expected identifier to start with ascii letter, not '1' at position 7"
-    );
+    let config = parse_conf_str(input)?;
+    assert_eq!(config.service(), "https");
+    let mut expected = HashMap::new();
+    expected.insert("123".to_string(), "456".to_string());
+    assert_eq!(config.params(), &expected);
+    Ok(())
+}
+
+#[test]
+fn identifiers_can_contain_underscores() -> Result<(), ParsingError> {
+    let input = "_A_::__x_Y__=42;";
+    let config = parse_conf_str(input)?;
+    assert_eq!(config.service(), "_a_");
+    let mut expected = HashMap::new();
+    expected.insert("__x_y__".to_string(), "42".to_string());
+    Ok(())
 }
 
 #[test]