diff --git a/src/constant.js b/src/constant.js
index 22d6e8adc..ae555891e 100644
--- a/src/constant.js
+++ b/src/constant.js
@@ -27,4 +27,4 @@ export const INVALID_DATE_STRING = 'Invalid Date'
 
 // regex
 export const REGEX_PARSE = /^(\d{4})[-/]?(\d{1,2})?[-/]?(\d{0,2})[Tt\s]*(\d{1,2})?:?(\d{1,2})?:?(\d{1,2})?[.:]?(\d+)?$/
-export const REGEX_FORMAT = /\[([^\]]+)]|Y{1,4}|M{1,4}|D{1,2}|d{1,4}|H{1,2}|h{1,2}|a|A|m{1,2}|s{1,2}|Z{1,2}|SSS/g
+export const REGEX_FORMAT = /(?![^\[\]])\[([^\[\]]+)]|Y{1,4}|M{1,4}|D{1,2}|d{1,4}|H{1,2}|h{1,2}|a|A|m{1,2}|s{1,2}|Z{1,2}|SSS/g
diff --git a/src/plugin/localizedFormat/utils.js b/src/plugin/localizedFormat/utils.js
index b6284d8e9..d002a954b 100644
--- a/src/plugin/localizedFormat/utils.js
+++ b/src/plugin/localizedFormat/utils.js
@@ -1,6 +1,6 @@
 // eslint-disable-next-line import/prefer-default-export
 export const t = format =>
-  format.replace(/(\[[^\]]+])|(MMMM|MM|DD|dddd)/g, (_, a, b) => a || b.slice(1))
+  format.replace(/(\[(?!\[)[^\]]+])|(MMMM|MM|DD|dddd)/g, (_, a, b) => a || b.slice(1))
 
 export const englishFormats = {
   LTS: 'h:mm:ss A',
@@ -11,7 +11,7 @@ export const englishFormats = {
   LLLL: 'dddd, MMMM D, YYYY h:mm A'
 }
 
-export const u = (formatStr, formats) => formatStr.replace(/(\[[^\]]+])|(LTS?|l{1,4}|L{1,4})/g, (_, a, b) => {
+export const u = (formatStr, formats) => formatStr.replace(/(\[(?!\[)[^\]]+])|(LTS?|l{1,4}|L{1,4})/g, (_, a, b) => {
   const B = b && b.toUpperCase()
   return a || formats[b] || englishFormats[b] || t(formats[B])
 })
diff --git a/test/plugin/localizedFormat.test.js b/test/plugin/localizedFormat.test.js
index da5b219e0..3fe16ce36 100644
--- a/test/plugin/localizedFormat.test.js
+++ b/test/plugin/localizedFormat.test.js
@@ -116,3 +116,12 @@ it('Uses the localized uppercase formats as a base for lowercase formats, if not
     expect(spanishDate.format(option)).toBe(spanishDate.format(adaptedFormat))
   })
 })
+
+it('ReDos attack', () => {
+  const longFmt = '['.repeat(100000) + '\u0000'
+  const start = Date.now()
+  const output = dayjs().format(longFmt)
+  const elapsed = Date.now() - start
+  expect(output).toBe(longFmt)
+  expect(elapsed).toBeLessThan(3000)
+})