From 6d497055c303f1f50d31e595b238eb74fbe01120 Mon Sep 17 00:00:00 2001
From: brontybuutveld <brontybuutveld@gmail.com>
Date: Sun, 15 Feb 2026 19:25:06 +1300
Subject: [PATCH] feat: modifier support

---
 lib/reconstruct.ts  | 10 +++++++-
 lib/tokenizer.ts    | 61 +++++++++++++++++++++++++++++++++++++++++++++
 lib/types/tokens.ts |  3 +++
 3 files changed, 73 insertions(+), 1 deletion(-)

diff --git a/lib/reconstruct.ts b/lib/reconstruct.ts
index 274cd22..93b5d93 100644
--- a/lib/reconstruct.ts
+++ b/lib/reconstruct.ts
@@ -41,7 +41,15 @@ export const reconstruct = (token: Tokens): string => {
             token.followedBy ? '?=' :
               token.notFollowedBy ? '?!' :
                 '?:';
-      return `(${prefix}${createAlternate(token)})`;
+      let preprefix = '';
+      if (token.enableStack || token.disableStack) {
+        preprefix += '?';
+        if (token.enableStack) preprefix += reduceStack(token.enableStack);
+        if (token.dash) preprefix += '-';
+        if (token.disableStack) preprefix += reduceStack(token.disableStack);
+        preprefix += ':';
+      }
+      return `(${preprefix+prefix}${createAlternate(token)})`;
     }
     case types.REPETITION: {
       const { min, max } = token;
diff --git a/lib/tokenizer.ts b/lib/tokenizer.ts
index 34a522a..3ae62f9 100644
--- a/lib/tokenizer.ts
+++ b/lib/tokenizer.ts
@@ -14,6 +14,11 @@ const captureGroupFirstChar = /^[a-zA-Z_$]$/i;
  */
 const captureGroupChars = /^[a-zA-Z0-9_$]$/i;
 
+/**
+ * Valid characters for group modifiers.
+ */
+const modeModifiers = /^(i|m|s)$/i;
+
 const digit = /\d/;
 
 /**
@@ -42,6 +47,42 @@ export const tokenizer = (regexpStr: string): Root => {
     );
   };
 
+  const modifierErr = (idx: number) => {
+    throw new SyntaxError(
+      `Invalid regular expression: /${
+        regexpStr
+      }/: Unknown group modifier flag, expected 'i', or 'm', or 's', found` +
+      ` '${str[idx]}' at column ${idx + 1}`,
+    );
+  }
+
+  const tokenizeModifiers = (group: Group, i: number, enable: boolean): number => {
+    group[enable ? 'enableStack' : 'disableStack'] = [];
+
+    while (i < str.length && modeModifiers.test(str[i])) {
+      const char: Char = {
+        type: types.CHAR,
+        value: str[i].charCodeAt(0),
+      };
+      group[enable ? 'enableStack' : 'disableStack'].push(char);
+      i++;
+    }
+    
+    if (str[i] !== ':' && ((str[i] !== '-' && enable) || (!enable))) {
+      modifierErr(i);
+    }
+
+    return i;
+  }
+
+  const enableModifers = (group: Group, i: number): number => {
+    return tokenizeModifiers(group, i, true);
+  }
+
+  const disableModifers = (group: Group, i: number): number => {
+    return tokenizeModifiers(group, i, false);
+  }
+
   // Decode a few escaped characters.
   let str = util.strToChars(regexpStr);
 
@@ -222,6 +263,26 @@ export const tokenizer = (regexpStr: string): Root => {
             i++;
           } else if (c === ':') {
             group.remember = false;
+          } else if (modeModifiers.test(c)) {
+            // the index logic is complicated
+            i = enableModifers(group, --i); // i is ahead of c so we decrement
+            if (str[i] === '-') {
+              group.dash = true;
+              i = disableModifers(group, ++i) +1; // increment move off of '-', add 1 move index
+              if (group.disableStack.length === 0) {
+                group.disableStack = undefined;
+              }
+            } else if (str[i] === ':') { // like (?i:)
+              i++;
+            } else {
+              modifierErr(i);
+            }
+          } else if (c === '-') {
+            group.dash = true;
+            i = disableModifers(group, i) +1; // no increment here because i is ahead of c
+            if (group.disableStack.length === 0) {
+              group.disableStack = undefined;
+            }
           } else {
             throw new SyntaxError(
               `Invalid regular expression: /${
diff --git a/lib/types/tokens.ts b/lib/types/tokens.ts
index 9acb7ae..4bf2c41 100644
--- a/lib/types/tokens.ts
+++ b/lib/types/tokens.ts
@@ -18,6 +18,9 @@ export type Group = Base<types.GROUP, {
   notFollowedBy?: boolean;
   lookBehind?: boolean;
   name?: string;
+  enableStack?: Token[];
+  disableStack?: Token[];
+  dash?: boolean;
 }>
 
 export type Set = Base<types.SET, {