diff --git a/python/ql/lib/change-notes/2024-01-17-regex-flag-parsing.md b/python/ql/lib/change-notes/2024-01-17-regex-flag-parsing.md
new file mode 100644
index 000000000000..532ab1a88dc0
--- /dev/null
+++ b/python/ql/lib/change-notes/2024-01-17-regex-flag-parsing.md
@@ -0,0 +1,4 @@
+---
+category: fix
+---
+* Fixed regular expressions containing flags not being parsed correctly in some cases.
diff --git a/python/ql/lib/semmle/python/regexp/internal/ParseRegExp.qll b/python/ql/lib/semmle/python/regexp/internal/ParseRegExp.qll
index 90a976824ac3..6a70108c7f9c 100644
--- a/python/ql/lib/semmle/python/regexp/internal/ParseRegExp.qll
+++ b/python/ql/lib/semmle/python/regexp/internal/ParseRegExp.qll
@@ -116,13 +116,14 @@ class RegExp extends Expr instanceof StrConst {
 
   /**
    * Gets a mode (if any) of this regular expression. Can be any of:
-   * DEBUG
-   * IGNORECASE
-   * LOCALE
-   * MULTILINE
-   * DOTALL
-   * UNICODE
-   * VERBOSE
+   * - DEBUG
+   * - ASCII
+   * - IGNORECASE
+   * - LOCALE
+   * - MULTILINE
+   * - DOTALL
+   * - UNICODE
+   * - VERBOSE
    */
   string getAMode() {
     result = FindRegexMode::getAMode(this)
@@ -695,7 +696,10 @@ class RegExp extends Expr instanceof StrConst {
    */
   private predicate flag_group_start(int start, int end) {
     this.flag_group_start_no_modes(start, _) and
-    end = max(int i | this.mode_character(start, i) | i + 1)
+    // Check if this is a group with flags, and therefore the `:` should be excluded
+    exists(int maybe_end | maybe_end = max(int i | this.mode_character(start, i) | i + 1) |
+      if this.getChar(maybe_end) = ":" then end = maybe_end + 1 else end = maybe_end
+    )
   }
 
   /**
@@ -705,19 +709,21 @@ class RegExp extends Expr instanceof StrConst {
   private predicate flag_group_start_no_modes(int start, int end) {
     this.isGroupStart(start) and
     this.getChar(start + 1) = "?" and
-    this.getChar(start + 2) in ["i", "L", "m", "s", "u", "x"] and
+    // "-" is for removing flags, e.g. `(?-i:text)`
+    this.getChar(start + 2) in ["-", "a", "i", "L", "m", "s", "u", "x"] and
     end = start + 2
   }
 
   /**
-   * Holds if `pos` contains a mo character from the
+   * Holds if `pos` contains a mode character from the
    * flag group starting at `start`.
    */
   private predicate mode_character(int start, int pos) {
     this.flag_group_start_no_modes(start, pos)
     or
     this.mode_character(start, pos - 1) and
-    this.getChar(pos) in ["i", "L", "m", "s", "u", "x"]
+    // "-" is for removing flags, e.g. `(?-i:text)`
+    this.getChar(pos) in ["-", "a", "i", "L", "m", "s", "u", "x"]
   }
 
   /**
@@ -728,9 +734,13 @@ class RegExp extends Expr instanceof StrConst {
    * ```
    */
   private predicate flag(string c) {
-    exists(int pos |
-      this.mode_character(_, pos) and
-      this.getChar(pos) = c
+    exists(int start, int pos |
+      this.mode_character(start, pos) and
+      this.getChar(pos) = c and
+      // Ignore if flag is removed using `-`; use `<=` to also exclude `-` itself
+      not exists(int minus_pos |
+        this.mode_character(start, minus_pos) and this.getChar(minus_pos) = "-" and minus_pos <= pos
+      )
     )
   }
 
@@ -740,6 +750,8 @@ class RegExp extends Expr instanceof StrConst {
    */
   string getModeFromPrefix() {
     exists(string c | this.flag(c) |
+      c = "a" and result = "ASCII"
+      or
       c = "i" and result = "IGNORECASE"
       or
       c = "L" and result = "LOCALE"
diff --git a/python/ql/test/library-tests/regex/Characters.expected b/python/ql/test/library-tests/regex/Characters.expected
index 7a1ca9c874fa..dcbdd6b7d7ca 100644
--- a/python/ql/test/library-tests/regex/Characters.expected
+++ b/python/ql/test/library-tests/regex/Characters.expected
@@ -20,6 +20,7 @@
 | (?!not-this)^[A-Z_]+$ | 16 | 17 |
 | (?!not-this)^[A-Z_]+$ | 17 | 18 |
 | (?!not-this)^[A-Z_]+$ | 20 | 21 |
+| (?-imsx:a+) | 8 | 9 |
 | (?:(?:\n\r?)\|^)( *)\\S | 6 | 7 |
 | (?:(?:\n\r?)\|^)( *)\\S | 7 | 8 |
 | (?:(?:\n\r?)\|^)( *)\\S | 11 | 12 |
@@ -35,9 +36,14 @@
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | 19 | 21 |
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | 22 | 23 |
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | 24 | 25 |
+| (?Li)a+ | 5 | 6 |
 | (?P<name>[\\w]+)\| | 10 | 12 |
+| (?a-imsx:a+) | 9 | 10 |
+| (?aimsx)a+ | 8 | 9 |
+| (?aimsx:a+) | 8 | 9 |
 | (?m)^(?!$) | 4 | 5 |
 | (?m)^(?!$) | 8 | 9 |
+| (?ui)a+ | 5 | 6 |
 | (\\033\|~{) | 1 | 5 |
 | (\\033\|~{) | 6 | 7 |
 | (\\033\|~{) | 7 | 8 |
diff --git a/python/ql/test/library-tests/regex/FirstLast.expected b/python/ql/test/library-tests/regex/FirstLast.expected
index e388e0d1fdf7..43695539c0b7 100644
--- a/python/ql/test/library-tests/regex/FirstLast.expected
+++ b/python/ql/test/library-tests/regex/FirstLast.expected
@@ -7,6 +7,10 @@
 | (?!not-this)^[A-Z_]+$ | last | 13 | 19 |
 | (?!not-this)^[A-Z_]+$ | last | 13 | 20 |
 | (?!not-this)^[A-Z_]+$ | last | 20 | 21 |
+| (?-imsx:a+) | first | 8 | 9 |
+| (?-imsx:a+) | first | 8 | 10 |
+| (?-imsx:a+) | last | 8 | 9 |
+| (?-imsx:a+) | last | 8 | 10 |
 | (?:(?:\n\r?)\|^)( *)\\S | first | 6 | 7 |
 | (?:(?:\n\r?)\|^)( *)\\S | first | 11 | 12 |
 | (?:(?:\n\r?)\|^)( *)\\S | last | 17 | 19 |
@@ -18,14 +22,34 @@
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | first | 8 | 9 |
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | first | 11 | 12 |
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | last | 21 | 26 |
+| (?Li)a+ | first | 5 | 6 |
+| (?Li)a+ | first | 5 | 7 |
+| (?Li)a+ | last | 5 | 6 |
+| (?Li)a+ | last | 5 | 7 |
 | (?P<name>[\\w]+)\| | first | 9 | 13 |
 | (?P<name>[\\w]+)\| | first | 9 | 14 |
 | (?P<name>[\\w]+)\| | last | 9 | 13 |
 | (?P<name>[\\w]+)\| | last | 9 | 14 |
+| (?a-imsx:a+) | first | 9 | 10 |
+| (?a-imsx:a+) | first | 9 | 11 |
+| (?a-imsx:a+) | last | 9 | 10 |
+| (?a-imsx:a+) | last | 9 | 11 |
+| (?aimsx)a+ | first | 8 | 9 |
+| (?aimsx)a+ | first | 8 | 10 |
+| (?aimsx)a+ | last | 8 | 9 |
+| (?aimsx)a+ | last | 8 | 10 |
+| (?aimsx:a+) | first | 8 | 9 |
+| (?aimsx:a+) | first | 8 | 10 |
+| (?aimsx:a+) | last | 8 | 9 |
+| (?aimsx:a+) | last | 8 | 10 |
 | (?m)^(?!$) | first | 4 | 5 |
 | (?m)^(?!$) | first | 8 | 9 |
 | (?m)^(?!$) | last | 4 | 5 |
 | (?m)^(?!$) | last | 8 | 9 |
+| (?ui)a+ | first | 5 | 6 |
+| (?ui)a+ | first | 5 | 7 |
+| (?ui)a+ | last | 5 | 6 |
+| (?ui)a+ | last | 5 | 7 |
 | (\\033\|~{) | first | 1 | 5 |
 | (\\033\|~{) | first | 6 | 8 |
 | (\\033\|~{) | last | 1 | 5 |
diff --git a/python/ql/test/library-tests/regex/GroupContents.expected b/python/ql/test/library-tests/regex/GroupContents.expected
index c7c4ac97a1e3..0e2ca7cda612 100644
--- a/python/ql/test/library-tests/regex/GroupContents.expected
+++ b/python/ql/test/library-tests/regex/GroupContents.expected
@@ -1,4 +1,5 @@
 | (?!not-this)^[A-Z_]+$ | 0 | 12 | (?!not-this) | 3 | 11 | not-this |
+| (?-imsx:a+) | 0 | 11 | (?-imsx:a+) | 8 | 10 | a+ |
 | (?:(?:\n\r?)\|^)( *)\\S | 0 | 13 | (?:(?:\n\r?)\|^) | 3 | 12 | (?:\n\r?)\|^ |
 | (?:(?:\n\r?)\|^)( *)\\S | 3 | 10 | (?:\n\r?) | 6 | 9 | \n\r? |
 | (?:(?:\n\r?)\|^)( *)\\S | 13 | 17 | ( *) | 14 | 16 |  * |
@@ -8,6 +9,8 @@
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | 0 | 10 | (?:[^%]\|^) | 3 | 9 | [^%]\|^ |
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | 14 | 19 | (\\w*) | 15 | 18 | \\w* |
 | (?P<name>[\\w]+)\| | 0 | 15 | (?P<name>[\\w]+) | 9 | 14 | [\\w]+ |
+| (?a-imsx:a+) | 0 | 12 | (?a-imsx:a+) | 9 | 11 | a+ |
+| (?aimsx:a+) | 0 | 11 | (?aimsx:a+) | 8 | 10 | a+ |
 | (?m)^(?!$) | 5 | 10 | (?!$) | 8 | 9 | $ |
 | (\\033\|~{) | 0 | 9 | (\\033\|~{) | 1 | 8 | \\033\|~{ |
 | \\[(?P<txt>[^[]*)\\]\\((?P<uri>[^)]*) | 2 | 16 | (?P<txt>[^[]*) | 10 | 15 | [^[]* |
diff --git a/python/ql/test/library-tests/regex/Mode.expected b/python/ql/test/library-tests/regex/Mode.expected
index 3fcfc8672c1b..eeb2c911214e 100644
--- a/python/ql/test/library-tests/regex/Mode.expected
+++ b/python/ql/test/library-tests/regex/Mode.expected
@@ -10,4 +10,19 @@
 | 54 | DOTALL |
 | 54 | VERBOSE |
 | 56 | VERBOSE |
-| 68 | MULTILINE |
+| 59 | ASCII |
+| 59 | DOTALL |
+| 59 | IGNORECASE |
+| 59 | MULTILINE |
+| 59 | VERBOSE |
+| 60 | IGNORECASE |
+| 60 | UNICODE |
+| 61 | IGNORECASE |
+| 61 | LOCALE |
+| 62 | ASCII |
+| 62 | DOTALL |
+| 62 | IGNORECASE |
+| 62 | MULTILINE |
+| 62 | VERBOSE |
+| 64 | ASCII |
+| 76 | MULTILINE |
diff --git a/python/ql/test/library-tests/regex/Qualified.expected b/python/ql/test/library-tests/regex/Qualified.expected
index b698a1d09557..01540b83c55b 100644
--- a/python/ql/test/library-tests/regex/Qualified.expected
+++ b/python/ql/test/library-tests/regex/Qualified.expected
@@ -1,9 +1,15 @@
 | (?!not-this)^[A-Z_]+$ | 13 | 20 | false | true |
+| (?-imsx:a+) | 8 | 10 | false | true |
 | (?:(?:\n\r?)\|^)( *)\\S | 7 | 9 | true | false |
 | (?:(?:\n\r?)\|^)( *)\\S | 14 | 16 | true | true |
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | 0 | 11 | true | false |
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | 15 | 18 | true | true |
+| (?Li)a+ | 5 | 7 | false | true |
 | (?P<name>[\\w]+)\| | 9 | 14 | false | true |
+| (?a-imsx:a+) | 9 | 11 | false | true |
+| (?aimsx)a+ | 8 | 10 | false | true |
+| (?aimsx:a+) | 8 | 10 | false | true |
+| (?ui)a+ | 5 | 7 | false | true |
 | \\A[+-]?\\d+ | 2 | 7 | true | false |
 | \\A[+-]?\\d+ | 7 | 10 | false | true |
 | \\[(?P<txt>[^[]*)\\]\\((?P<uri>[^)]*) | 10 | 15 | true | true |
diff --git a/python/ql/test/library-tests/regex/Regex.expected b/python/ql/test/library-tests/regex/Regex.expected
index e5e0ea1719ec..be32d3a4da37 100644
--- a/python/ql/test/library-tests/regex/Regex.expected
+++ b/python/ql/test/library-tests/regex/Regex.expected
@@ -26,6 +26,10 @@
 | (?!not-this)^[A-Z_]+$ | qualified | 13 | 20 |
 | (?!not-this)^[A-Z_]+$ | sequence | 0 | 21 |
 | (?!not-this)^[A-Z_]+$ | sequence | 3 | 11 |
+| (?-imsx:a+) | char | 8 | 9 |
+| (?-imsx:a+) | non-empty group | 0 | 11 |
+| (?-imsx:a+) | qualified | 8 | 10 |
+| (?-imsx:a+) | sequence | 0 | 11 |
 | (?:(?:\n\r?)\|^)( *)\\S | ^ | 11 | 12 |
 | (?:(?:\n\r?)\|^)( *)\\S | char | 6 | 7 |
 | (?:(?:\n\r?)\|^)( *)\\S | char | 7 | 8 |
@@ -69,18 +73,38 @@
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | sequence | 0 | 26 |
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | sequence | 3 | 7 |
 | (?:[^%]\|^)?%\\((\\w*)\\)[a-z] | sequence | 8 | 9 |
+| (?Li)a+ | char | 5 | 6 |
+| (?Li)a+ | empty group | 0 | 5 |
+| (?Li)a+ | qualified | 5 | 7 |
+| (?Li)a+ | sequence | 0 | 7 |
 | (?P<name>[\\w]+)\| | char | 10 | 12 |
 | (?P<name>[\\w]+)\| | char-set | 9 | 13 |
 | (?P<name>[\\w]+)\| | choice | 0 | 16 |
 | (?P<name>[\\w]+)\| | non-empty group | 0 | 15 |
 | (?P<name>[\\w]+)\| | qualified | 9 | 14 |
 | (?P<name>[\\w]+)\| | sequence | 0 | 15 |
+| (?a-imsx:a+) | char | 9 | 10 |
+| (?a-imsx:a+) | non-empty group | 0 | 12 |
+| (?a-imsx:a+) | qualified | 9 | 11 |
+| (?a-imsx:a+) | sequence | 0 | 12 |
+| (?aimsx)a+ | char | 8 | 9 |
+| (?aimsx)a+ | empty group | 0 | 8 |
+| (?aimsx)a+ | qualified | 8 | 10 |
+| (?aimsx)a+ | sequence | 0 | 10 |
+| (?aimsx:a+) | char | 8 | 9 |
+| (?aimsx:a+) | non-empty group | 0 | 11 |
+| (?aimsx:a+) | qualified | 8 | 10 |
+| (?aimsx:a+) | sequence | 0 | 11 |
 | (?m)^(?!$) | $ | 8 | 9 |
 | (?m)^(?!$) | ^ | 4 | 5 |
 | (?m)^(?!$) | empty group | 0 | 4 |
 | (?m)^(?!$) | empty group | 5 | 10 |
 | (?m)^(?!$) | sequence | 0 | 10 |
 | (?m)^(?!$) | sequence | 8 | 9 |
+| (?ui)a+ | char | 5 | 6 |
+| (?ui)a+ | empty group | 0 | 5 |
+| (?ui)a+ | qualified | 5 | 7 |
+| (?ui)a+ | sequence | 0 | 7 |
 | (\\033\|~{) | char | 1 | 5 |
 | (\\033\|~{) | char | 6 | 7 |
 | (\\033\|~{) | char | 7 | 8 |
diff --git a/python/ql/test/library-tests/regex/test.py b/python/ql/test/library-tests/regex/test.py
index b8e6daea7e6e..713f69b7270c 100644
--- a/python/ql/test/library-tests/regex/test.py
+++ b/python/ql/test/library-tests/regex/test.py
@@ -55,6 +55,14 @@
 # re.X is an alias for re.VERBOSE
 re.compile("", re.X)
 
+#Inline flags; 'a', 'L' and 'u' are mutually exclusive
+re.compile("(?aimsx)a+")
+re.compile("(?ui)a+")
+re.compile(b"(?Li)a+")
+re.compile("(?aimsx:a+)")
+re.compile("(?-imsx:a+)")
+re.compile("(?a-imsx:a+)")
+
 #empty choice
 re.compile(r'|x')
 re.compile(r'x|')