perf: Optimize hot paths in utility functions

packages/tailwindcss/src/canonicalize-candidates.ts

@@ -1099,10 +1099,14 @@ function allVariablesAreUsed(
   walk(ValueParser.parse(value), (node) => {
     if (node.kind === 'function' && node.value === 'var') {
       let variable = node.nodes[0].value
-      let r = new RegExp(`var\\(${variable}[,)]\\s*`, 'g')
+      // Check if the variable is used in the replacement using string methods
+      // instead of RegExp to avoid the overhead of creating a RegExp per variable
+      let hasVarWithComma = replacementAsCss.includes(`var(${variable},`)
+      let hasVarWithParen = replacementAsCss.includes(`var(${variable})`)
+
       if (
         // We need to check if the variable is used in the replacement
-        !r.test(replacementAsCss) ||
+        !(hasVarWithComma || hasVarWithParen) ||
         // The value cannot be set to a different value in the
         // replacement because that would make it an unsafe migration
         replacementAsCss.includes(`${variable}:`)

packages/tailwindcss/src/utils/brace-expansion.ts

@@ -68,24 +68,25 @@ function expandSequence(seq: string): string[] {
   let step = stepStr ? parseInt(stepStr, 10) : undefined
   let result: string[] = []
 
-  if (/^-?\d+$/.test(start) && /^-?\d+$/.test(end)) {
-    let startNum = parseInt(start, 10)
-    let endNum = parseInt(end, 10)
+  // NUMERICAL_RANGE already ensures start and end are numbers (-?\d+)
+  // so no need to test again
+  let startNum = parseInt(start, 10)
+  let endNum = parseInt(end, 10)
 
-    if (step === undefined) {
-      step = startNum <= endNum ? 1 : -1
-    }
-    if (step === 0) {
-      throw new Error('Step cannot be zero in sequence expansion.')
-    }
+  if (step === undefined) {
+    step = startNum <= endNum ? 1 : -1
+  }
+  if (step === 0) {
+    throw new Error('Step cannot be zero in sequence expansion.')
+  }
 
-    let increasing = startNum < endNum
-    if (increasing && step < 0) step = -step
-    if (!increasing && step > 0) step = -step
+  let increasing = startNum < endNum
+  if (increasing && step < 0) step = -step
+  if (!increasing && step > 0) step = -step
 
-    for (let i = startNum; increasing ? i <= endNum : i >= endNum; i += step) {
-      result.push(i.toString())
-    }
+  for (let i = startNum; increasing ? i <= endNum : i >= endNum; i += step) {
+    result.push(i.toString())
   }
+
   return result
 }

packages/tailwindcss/src/utils/decode-arbitrary-value.ts

@@ -21,6 +21,11 @@ export function decodeArbitraryValue(input: string): string {
  * converted to `_` instead.
  */
 function convertUnderscoresToWhitespace(input: string, skipUnderscoreToSpace = false) {
+  // Fast path: if there are no underscores at all, return input as-is
+  if (input.indexOf('_') === -1) {
+    return input
+  }
+
   let output = ''
   for (let i = 0; i < input.length; i++) {
     let char = input[i]

packages/tailwindcss/src/utils/infer-data-type.ts

@@ -66,14 +66,11 @@ function isUrl(value: string): boolean {
 
 /* -------------------------------------------------------------------------- */
 
+const LINE_WIDTH_KEYWORDS = new Set(['thin', 'medium', 'thick'])
+
 function isLineWidth(value: string): boolean {
   return segment(value, ' ').every(
-    (value) =>
-      isLength(value) ||
-      isNumber(value) ||
-      value === 'thin' ||
-      value === 'medium' ||
-      value === 'thick',
+    (value) => isLength(value) || isNumber(value) || LINE_WIDTH_KEYWORDS.has(value),
   )
 }
 
@@ -111,22 +108,24 @@ function isImage(value: string) {
 
 /* -------------------------------------------------------------------------- */
 
+const GENERIC_NAMES = new Set([
+  'serif',
+  'sans-serif',
+  'monospace',
+  'cursive',
+  'fantasy',
+  'system-ui',
+  'ui-serif',
+  'ui-sans-serif',
+  'ui-monospace',
+  'ui-rounded',
+  'math',
+  'emoji',
+  'fangsong',
+])
+
 function isGenericName(value: string): boolean {
-  return (
-    value === 'serif' ||
-    value === 'sans-serif' ||
-    value === 'monospace' ||
-    value === 'cursive' ||
-    value === 'fantasy' ||
-    value === 'system-ui' ||
-    value === 'ui-serif' ||
-    value === 'ui-sans-serif' ||
-    value === 'ui-monospace' ||
-    value === 'ui-rounded' ||
-    value === 'math' ||
-    value === 'emoji' ||
-    value === 'fangsong'
-  )
+  return GENERIC_NAMES.has(value)
 }
 
 function isFamilyName(value: string): boolean {
@@ -145,17 +144,19 @@ function isFamilyName(value: string): boolean {
   return count > 0
 }
 
+const ABSOLUTE_SIZES = new Set([
+  'xx-small',
+  'x-small',
+  'small',
+  'medium',
+  'large',
+  'x-large',
+  'xx-large',
+  'xxx-large',
+])
+
 function isAbsoluteSize(value: string): boolean {
-  return (
-    value === 'xx-small' ||
-    value === 'x-small' ||
-    value === 'small' ||
-    value === 'medium' ||
-    value === 'large' ||
-    value === 'x-large' ||
-    value === 'xx-large' ||
-    value === 'xxx-large'
-  )
+  return ABSOLUTE_SIZES.has(value)
 }
 
 function isRelativeSize(value: string): boolean {
@@ -239,17 +240,13 @@ export function isLength(value: string): boolean {
 
 /* -------------------------------------------------------------------------- */
 
+const BACKGROUND_POSITION_KEYWORDS = new Set(['center', 'top', 'right', 'bottom', 'left'])
+
 function isBackgroundPosition(value: string): boolean {
   let count = 0
 
   for (let part of segment(value, ' ')) {
-    if (
-      part === 'center' ||
-      part === 'top' ||
-      part === 'right' ||
-      part === 'bottom' ||
-      part === 'left'
-    ) {
+    if (BACKGROUND_POSITION_KEYWORDS.has(part)) {
       count += 1
       continue
     }

packages/tailwindcss/src/utils/is-color.ts

@@ -198,7 +198,26 @@ const NAMED_COLORS = new Set([
 const IS_COLOR_FN = /^(rgba?|hsla?|hwb|color|(ok)?(lab|lch)|light-dark|color-mix)\(/i
 
 export function isColor(value: string): boolean {
-  return (
-    value.charCodeAt(0) === HASH || IS_COLOR_FN.test(value) || NAMED_COLORS.has(value.toLowerCase())
-  )
+  // Fast path: check for hash first (hex colors)
+  if (value.charCodeAt(0) === HASH) return true
+
+  // Fast path: check for color functions
+  if (IS_COLOR_FN.test(value)) return true
+
+  // Check named colors - try lowercase first, then convert if needed
+  // Most values in practice are already lowercase
+  if (NAMED_COLORS.has(value)) return true
+
+  // Only convert to lowercase if the first check failed and value contains uppercase
+  // This avoids the toLowerCase() call in the common case
+  let hasUppercase = false
+  for (let i = 0; i < value.length; i++) {
+    let code = value.charCodeAt(i)
+    if (code >= 65 && code <= 90) { // A-Z
+      hasUppercase = true
+      break
+    }
+  }
+
+  return hasUppercase && NAMED_COLORS.has(value.toLowerCase())
 }

packages/tailwindcss/src/utils/math-operators.ts

@@ -39,12 +39,44 @@ const MATH_FUNCTIONS = [
 ]
 
 export function hasMathFn(input: string) {
-  return input.indexOf('(') !== -1 && MATH_FUNCTIONS.some((fn) => input.includes(`${fn}(`))
+  // Fast path: no opening paren means no function calls
+  if (input.indexOf('(') === -1) return false
+
+  // Check for math functions without creating template strings
+  for (let fn of MATH_FUNCTIONS) {
+    // Build the pattern to match: "fn("
+    // Look for the function name followed immediately by opening paren
+    let idx = 0
+    while ((idx = input.indexOf(fn, idx)) !== -1) {
+      // Check if the character after the function name is '('
+      // charCodeAt returns NaN for out-of-bounds, which won't equal OPEN_PAREN
+      if (input.charCodeAt(idx + fn.length) === OPEN_PAREN) {
+        return true
+      }
+      idx++
+    }
+  }
+
+  return false
 }
 
 export function addWhitespaceAroundMathOperators(input: string) {
   // Bail early if there are no math functions in the input
-  if (!MATH_FUNCTIONS.some((fn) => input.includes(fn))) {
+  // Check for opening paren first as a fast path
+  if (input.indexOf('(') === -1) {
+    return input
+  }
+
+  // Check if any math function exists in the string
+  let hasMathFunction = false
+  for (let fn of MATH_FUNCTIONS) {
+    if (input.includes(fn)) {
+      hasMathFunction = true
+      break
+    }
+  }
+
+  if (!hasMathFunction) {
     return input
   }
 

packages/tailwindcss/src/utils/to-key-path.ts

@@ -45,7 +45,7 @@ export function toKeyPath(path: string) {
     }
 
     // Add the part after the last bracket as a key
-    if (currentIndex <= part.length - 1) {
+    if (currentIndex < part.length) {
       keypath.push(part.slice(currentIndex))
     }
   }