diff --git a/java/ql/lib/change-notes/2023-07-20-regex-parse-modes.md b/java/ql/lib/change-notes/2023-07-20-regex-parse-modes.md
new file mode 100644
index 00000000000..2d676227491
--- /dev/null
+++ b/java/ql/lib/change-notes/2023-07-20-regex-parse-modes.md
@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* Regular expressions containing multiple parse mode flags are now interpretted correctly. For example `"(?is)abc.*"` with both the `i` and `s` flags.
diff --git a/java/ql/lib/change-notes/2023-09-12-regex-mode-flag-groups.md b/java/ql/lib/change-notes/2023-09-12-regex-mode-flag-groups.md
new file mode 100644
index 00000000000..d13350726a8
--- /dev/null
+++ b/java/ql/lib/change-notes/2023-09-12-regex-mode-flag-groups.md
@@ -0,0 +1,4 @@
+---
+category: fix
+---
+* The regular expressions library no longer incorrectly matches mode flag characters against the input.
diff --git a/java/ql/lib/semmle/code/java/regex/regex.qll b/java/ql/lib/semmle/code/java/regex/regex.qll
index aca51b74a03..a131ac0deb5 100644
--- a/java/ql/lib/semmle/code/java/regex/regex.qll
+++ b/java/ql/lib/semmle/code/java/regex/regex.qll
@@ -472,12 +472,48 @@ abstract class RegexString extends StringLiteral {
     )
   }
 
-  private predicate flagGroupStart(int start, int end, string c) {
+  /**
+   * Holds if the initial part of a parse mode, not containing any
+   * mode characters is between `start` and `end`.
+   */
+  private predicate flagGroupStartNoModes(int start, int end) {
     this.isGroupStart(start) and
     this.getChar(start + 1) = "?" and
-    end = start + 3 and
-    c = this.getChar(start + 2) and
-    c in ["i", "m", "s", "u", "x", "U"]
+    this.getChar(start + 2) in ["i", "m", "s", "u", "x", "U"] and
+    end = start + 2
+  }
+
+  /**
+   * Holds if `pos` contains a mode character from the
+   * flag group starting at `start`.
+   */
+  private predicate modeCharacter(int start, int pos) {
+    this.flagGroupStartNoModes(start, pos)
+    or
+    this.modeCharacter(start, pos - 1) and
+    this.getChar(pos) in ["i", "m", "s", "u", "x", "U"]
+  }
+
+  /**
+   * Holds if a parse mode group is between `start` and `end`.
+   */
+  private predicate flagGroupStart(int start, int end) {
+    this.flagGroupStartNoModes(start, _) and
+    end = max(int i | this.modeCharacter(start, i) | i + 1)
+  }
+
+  /**
+   * Holds if a parse mode group of this regex includes the mode flag `c`.
+   * For example the following parse mode group, with mode flag `i`:
+   * ```
+   * (?i)
+   * ```
+   */
+  private predicate flag(string c) {
+    exists(int pos |
+      this.modeCharacter(_, pos) and
+      this.getChar(pos) = c
+    )
   }
 
   /**
@@ -485,7 +521,7 @@ abstract class RegexString extends StringLiteral {
    * it is defined by a prefix.
    */
   string getModeFromPrefix() {
-    exists(string c | this.flagGroupStart(_, _, c) |
+    exists(string c | this.flag(c) |
       c = "i" and result = "IGNORECASE"
       or
       c = "m" and result = "MULTILINE"
@@ -540,7 +576,7 @@ abstract class RegexString extends StringLiteral {
   private predicate groupStart(int start, int end) {
     this.nonCapturingGroupStart(start, end)
     or
-    this.flagGroupStart(start, end, _)
+    this.flagGroupStart(start, end)
     or
     this.namedGroupStart(start, end)
     or
diff --git a/java/ql/test/query-tests/security/CWE-730/ExpRedosTest.java b/java/ql/test/query-tests/security/CWE-730/ExpRedosTest.java
index e7e876cb696..56017ee12e8 100644
--- a/java/ql/test/query-tests/security/CWE-730/ExpRedosTest.java
+++ b/java/ql/test/query-tests/security/CWE-730/ExpRedosTest.java
@@ -86,6 +86,9 @@ class ExpRedosTest {
         // NOT GOOD; attack: "\n".repeat(100) + "."
         "(?s)(.|\\n)*!", // $ hasExpRedos
 
+        // NOT GOOD; attack: "\n".repeat(100) + "."
+        "(?is)(.|\\n)*!", // $ hasExpRedos
+
         // GOOD
         "([\\w.]+)*",
 
@@ -120,7 +123,7 @@ class ExpRedosTest {
         "\"((?:\\\\[\\x00-\\x7f]|[^\\x00-\\x08\\x0a-\\x1f\\x7f\"])*)\"", // $ MISSING: hasExpRedos
 
         // GOOD
-        "\"((?:\\\\[\\x00-\\x7f]|[^\\x00-\\x08\\x0a-\\x1f\\x7f\"\\\\])*)\"", 
+        "\"((?:\\\\[\\x00-\\x7f]|[^\\x00-\\x08\\x0a-\\x1f\\x7f\"\\\\])*)\"",
 
         // NOT GOOD
         "(([a-z]|[d-h])*)\"", // $ hasExpRedos
@@ -428,7 +431,10 @@ class ExpRedosTest {
         "(a*)*b", // $ hasExpRedos
 
         // BAD - but not detected due to the way possessive quantifiers are approximated
-        "((aa|a*+)b)*c" // $ MISSING: hasExpRedos
+        "((aa|a*+)b)*c", // $ MISSING: hasExpRedos
+
+        // BAD - testing mode flag groups
+        "(?is)(a|aa?)*b" // $ hasExpRedos hasPrefixMsg= hasPump=a
     };
 
     void test() {
diff --git a/java/ql/test/query-tests/security/CWE-730/ReDoS.ql b/java/ql/test/query-tests/security/CWE-730/ReDoS.ql
index 97719364f94..4011946318e 100644
--- a/java/ql/test/query-tests/security/CWE-730/ReDoS.ql
+++ b/java/ql/test/query-tests/security/CWE-730/ReDoS.ql
@@ -4,8 +4,13 @@ private import semmle.code.java.regex.RegexTreeView::RegexTreeView as TreeView
 import codeql.regex.nfa.ExponentialBackTracking::Make<TreeView> as ExponentialBackTracking
 import semmle.code.java.regex.regex
 
+bindingset[s]
+string quote(string s) { if s.matches("% %") then result = "\"" + s + "\"" else result = s }
+
 module HasExpRedos implements TestSig {
-  string getARelevantTag() { result = ["hasExpRedos", "hasParseFailure"] }
+  string getARelevantTag() {
+    result = ["hasExpRedos", "hasParseFailure", "hasPump", "hasPrefixMsg"]
+  }
 
   predicate hasActualResult(Location location, string element, string tag, string value) {
     tag = "hasExpRedos" and
@@ -25,6 +30,22 @@ module HasExpRedos implements TestSig {
       element = r.toString()
     )
   }
+
+  predicate hasOptionalResult(Location location, string element, string tag, string value) {
+    exists(TreeView::RegExpTerm t, Regex r, string pump, string prefixMsg |
+      ExponentialBackTracking::hasReDoSResult(t, pump, _, prefixMsg) and
+      t.occursInRegex(r, _, _) and
+      (
+        tag = "hasPrefixMsg" and
+        value = quote(prefixMsg)
+        or
+        tag = "hasPump" and
+        value = pump
+      ) and
+      location = r.getLocation() and
+      element = r.toString()
+    )
+  }
 }
 
 import MakeTest<HasExpRedos>