diff --git a/bids-validator/src/schema/context.test.ts b/bids-validator/src/schema/context.test.ts
index c91d5bfed..958d6f9ca 100644
--- a/bids-validator/src/schema/context.test.ts
+++ b/bids-validator/src/schema/context.test.ts
@@ -1,113 +1,26 @@
 import { assert } from '../deps/asserts.ts'
 import { DatasetIssues } from '../issues/datasetIssues.ts'
-import { FileTree } from '../types/filetree.ts'
 import { BIDSContext } from './context.ts'
-import { nullReadBytes } from '../tests/nullReadBytes.ts'
-
-const issues = new DatasetIssues()
-
-const text = () => Promise.resolve('')
-
-const anatJson = () =>
-  Promise.resolve(
-    JSON.stringify({
-      rootOverwrite: 'anat',
-      subOverwrite: 'anat',
-      anatValue: 'anat',
-    }),
-  )
-const subjectJson = () =>
-  Promise.resolve(
-    JSON.stringify({ subOverwrite: 'subject', subValue: 'subject' }),
-  )
-const rootJson = () =>
-  Promise.resolve(JSON.stringify({ rootOverwrite: 'root', rootValue: 'root' }))
-
-const rootFileTree = new FileTree('/', '')
-const subjectFileTree = new FileTree('/sub-01', 'sub-01', rootFileTree)
-const sessionFileTree = new FileTree(
-  '/sub-01/ses-01',
-  'ses-01',
-  subjectFileTree,
-)
-const anatFileTree = new FileTree(
-  '/sub-01/ses-01/anat',
-  'anat',
-  sessionFileTree,
-)
-
-const dataFile = {
-  text,
-  path: '/sub-01/ses-01/anat/sub-01_ses-01_T1w.nii.gz',
-  name: 'sub-01_ses-01_T1w.nii.gz',
-  size: 311112,
-  ignored: false,
-  stream: new ReadableStream<Uint8Array>(),
-  readBytes: nullReadBytes,
-}
-
-anatFileTree.files = [
-  dataFile,
-  {
-    text: anatJson,
-    path: '/sub-01/ses-01/anat/sub-01_ses-01_T1w.json',
-    name: 'sub-01_ses-01_T1w.json',
-    size: 311112,
-    ignored: false,
-    stream: new ReadableStream<Uint8Array>(),
-    readBytes: nullReadBytes,
-  },
-]
-
-sessionFileTree.files = []
-sessionFileTree.directories = [anatFileTree]
-
-subjectFileTree.files = [
-  {
-    text: subjectJson,
-    path: '/sub-01/ses-01_T1w.json',
-    name: 'ses-01_T1w.json',
-    size: 311112,
-    ignored: false,
-    stream: new ReadableStream<Uint8Array>(),
-    readBytes: nullReadBytes,
-  },
-]
-subjectFileTree.directories = [sessionFileTree]
-
-rootFileTree.files = [
-  {
-    text: rootJson,
-    path: '/T1w.json',
-    name: 'T1w.json',
-    size: 311112,
-    ignored: false,
-    stream: new ReadableStream<Uint8Array>(),
-    readBytes: nullReadBytes,
-  },
-]
-rootFileTree.directories = [subjectFileTree]
-
-let context = new BIDSContext(anatFileTree, dataFile, issues)
+import { dataFile, rootFileTree } from './fixtures.test.ts'
 
 Deno.test('test context LoadSidecar', async (t) => {
+  const context = new BIDSContext(rootFileTree, dataFile, new DatasetIssues())
   await context.loadSidecar(rootFileTree)
   await t.step('sidecar overwrites correct fields', () => {
     const { rootOverwrite, subOverwrite } = context.sidecar
-    assert(rootOverwrite, 'anat')
-    assert(subOverwrite, 'anat')
+    assert(rootOverwrite === 'anat')
+    assert(subOverwrite === 'anat')
   })
   await t.step('sidecar adds new fields at each level', () => {
     const { rootValue, subValue, anatValue } = context.sidecar
-    assert(rootValue, 'root')
-    assert(subValue, 'subject')
-    assert(anatValue, 'anat')
+    assert(rootValue === 'root')
+    assert(subValue === 'subject')
+    assert(anatValue === 'anat')
   })
 })
 
-context = new BIDSContext(rootFileTree, dataFile, issues)
-
 Deno.test('test context loadSubjects', async (t) => {
+  const context = new BIDSContext(rootFileTree, dataFile, new DatasetIssues())
   await context.loadSubjects()
   await t.step('context produces correct subjects object', () => {
     assert(context.dataset.subjects, 'subjects object exists')
diff --git a/bids-validator/src/schema/expressionLanguage.test.ts b/bids-validator/src/schema/expressionLanguage.test.ts
new file mode 100644
index 000000000..0ca4b74c0
--- /dev/null
+++ b/bids-validator/src/schema/expressionLanguage.test.ts
@@ -0,0 +1,206 @@
+import { assert } from '../deps/asserts.ts'
+import { expressionFunctions } from './expressionLanguage.ts'
+import { dataFile, rootFileTree } from './fixtures.test.ts'
+import { BIDSContext } from './context.ts'
+import { DatasetIssues } from '../issues/datasetIssues.ts'
+
+Deno.test('test expression functions', async (t) => {
+  const context = new BIDSContext(rootFileTree, dataFile, new DatasetIssues())
+
+  await t.step('index function', () => {
+    const index = expressionFunctions.index
+    assert(index([1, 2, 3], 2) === 1)
+    assert(index([1, 2, 3], 4) === null)
+    assert(index(['a', 'b', 'c'], 'b') === 1)
+    assert(index(['a', 'b', 'c'], 'd') === null)
+  })
+  await t.step('intersects function', () => {
+    const intersects = expressionFunctions.intersects
+    assert(intersects([1, 2, 3], [2, 3, 4]) === true)
+    assert(intersects([1, 2, 3], [4, 5, 6]) === false)
+    assert(intersects(['abc', 'def'], ['def']) === true)
+    assert(intersects(['abc', 'def'], ['ghi']) === false)
+
+    // Promote scalars to arrays
+    // @ts-ignore
+    assert(intersects('abc', ['abc', 'def']) === true)
+    // @ts-ignore
+    assert(intersects('abc', ['a', 'b', 'c']) === false)
+  })
+  await t.step('match function', () => {
+    const match = expressionFunctions.match
+    assert(match('abc', 'abc') === true)
+    assert(match('abc', 'def') === false)
+    assert(match('abc', 'a.*') === true)
+    assert(match('abc', 'd.*') === false)
+  })
+  await t.step('type function', () => {
+    const type = expressionFunctions.type
+    assert(type(1) === 'number')
+    assert(type('abc') === 'string')
+    assert(type([1, 2, 3]) === 'array')
+    assert(type({ a: 1, b: 2 }) === 'object')
+    assert(type(true) === 'boolean')
+    assert(type(null) === 'null')
+    assert(type(undefined) === 'null')
+  })
+  await t.step('min function', () => {
+    const min = expressionFunctions.min
+    assert(min([1, 2, 3]) === 1)
+    assert(min([3, 2, 1]) === 1)
+    assert(min([]) === Infinity)
+    // @ts-ignore
+    assert(min(null) === null)
+  })
+  await t.step('max function', () => {
+    const max = expressionFunctions.max
+    assert(max([1, 2, 3]) === 3)
+    assert(max([3, 2, 1]) === 3)
+    assert(max([]) === -Infinity)
+    // @ts-ignore
+    assert(max(null) === null)
+  })
+  await t.step('length function', () => {
+    const length = expressionFunctions.length
+    assert(length([1, 2, 3]) === 3)
+    // Out-of-scope (but permitted) inputs
+    // @ts-ignore
+    assert(length('abc') === 3)
+    // Out-of-scope inputs
+    // @ts-ignore
+    assert(length({ a: 1, b: 2 }) === null)
+    // @ts-ignore
+    assert(length(true) === null)
+    // @ts-ignore
+    assert(length(null) === null)
+  })
+  await t.step('count function', () => {
+    const count = expressionFunctions.count
+    assert(count(['a', 'b', 'a', 'b'], 'a') === 2)
+    assert(count(['a', 'b', 'a', 'b'], 'c') === 0)
+  })
+  await t.step('exists(..., "dataset") function', () => {
+    const exists = expressionFunctions.exists.bind(context)
+    assert(exists([], 'dataset') === 0)
+    assert(
+      exists(['sub-01/ses-01/anat/sub-01_ses-01_T1w.nii.gz'], 'dataset') === 1,
+    )
+    assert(
+      exists(
+        ['sub-01/ses-01/anat/sub-01_ses-01_T1w.nii.gz', 'T1w.json'],
+        'dataset',
+      ) === 2,
+    )
+  })
+  await t.step('exists(..., "subject") function', () => {
+    const exists = expressionFunctions.exists.bind(context)
+    assert(exists([], 'subject') === 0)
+    assert(exists(['ses-01/anat/sub-01_ses-01_T1w.nii.gz'], 'subject') === 1)
+    assert(
+      exists(
+        ['ses-01/anat/sub-01_ses-01_T1w.nii.gz', 'T1w.json'],
+        'subject',
+      ) === 1,
+    )
+  })
+  await t.step('exists(..., "stimuli") function', () => {
+    const exists = expressionFunctions.exists.bind(context)
+    assert(exists([], 'stimuli') === 0)
+    assert(exists(['stimfile1.png'], 'stimuli') === 1)
+    assert(exists(['stimfile1.png', 'stimfile2.png'], 'stimuli') === 2)
+    assert(exists(['X.png', 'Y.png'], 'stimuli') === 0)
+  })
+  await t.step('exists(..., "bids-uri") function', () => {
+    const exists = expressionFunctions.exists.bind(context)
+    assert(exists([], 'subject') === 0)
+    assert(
+      exists(
+        ['bids::sub-01/ses-01/anat/sub-01_ses-01_T1w.nii.gz'],
+        'bids-uri',
+      ) === 1,
+    )
+    assert(
+      exists(
+        ['bids::sub-01/ses-01/anat/sub-01_ses-01_T1w.nii.gz', 'bids::T1w.json'],
+        'bids-uri',
+      ) === 2,
+    )
+    // Not yet implemented; currently returns length of array
+    // assert(exists(['bids::sub-01/ses-01/anat/sub-01_ses-01_T1w.nii.gz', 'bids::T2w.json'], 'bids-uri') === 1)
+  })
+  await t.step('substr function', () => {
+    const substr = expressionFunctions.substr
+    assert(substr('abc', 0, 1) === 'a')
+    assert(substr('abc', 1, 2) === 'b')
+    assert(substr('abc', 2, 3) === 'c')
+    assert(substr('abc', 3, 4) === '')
+    assert(substr('abc', 0, 4) === 'abc')
+    // @ts-ignore
+    assert(substr(null, 0, 1) === null)
+    // @ts-ignore
+    assert(substr('abc', null, 1) === null)
+    // @ts-ignore
+    assert(substr('abc', 0, null) === null)
+  })
+  await t.step('sorted(..., "numeric") function', () => {
+    const sorted = expressionFunctions.sorted
+    const array_equal = (a: any[], b: any[]) =>
+      a.length === b.length && a.every((v, i) => v === b[i])
+    assert(array_equal(sorted([3, 2, 1], 'numeric'), [1, 2, 3]))
+    assert(array_equal(sorted([1, 2, 3], 'numeric'), [1, 2, 3]))
+    assert(array_equal(sorted(['3', '2', '1'], 'numeric'), ['1', '2', '3']))
+    assert(array_equal(sorted(['1', '2', '3'], 'numeric'), ['1', '2', '3']))
+    assert(array_equal(sorted([], 'numeric'), []))
+    assert(array_equal(sorted([5, 25, 125, 625], 'numeric'), [5, 25, 125, 625]))
+    assert(array_equal(sorted([125, 25, 5, 625], 'numeric'), [5, 25, 125, 625]))
+  })
+  await t.step('sorted(..., "lexical") function', () => {
+    const sorted = expressionFunctions.sorted
+    const array_equal = (a: any[], b: any[]) =>
+      a.length === b.length && a.every((v, i) => v === b[i])
+    assert(array_equal(sorted([3, 2, 1], 'lexical'), [1, 2, 3]))
+    assert(array_equal(sorted([1, 2, 3], 'lexical'), [1, 2, 3]))
+    assert(array_equal(sorted(['3', '2', '1'], 'lexical'), ['1', '2', '3']))
+    assert(array_equal(sorted(['1', '2', '3'], 'lexical'), ['1', '2', '3']))
+    assert(array_equal(sorted([], 'lexical'), []))
+    assert(
+      array_equal(sorted(['5', '25', '125', '625'], 'lexical'), [
+        '125',
+        '25',
+        '5',
+        '625',
+      ]),
+    )
+    assert(
+      array_equal(sorted(['125', '25', '5', '625'], 'lexical'), [
+        '125',
+        '25',
+        '5',
+        '625',
+      ]),
+    )
+  })
+  await t.step('sorted(..., "auto") function', () => {
+    const sorted = expressionFunctions.sorted
+    const array_equal = (a: any[], b: any[]) =>
+      a.length === b.length && a.every((v, i) => v === b[i])
+    assert(array_equal(sorted([125, 25, 5, 625], 'auto'), [5, 25, 125, 625]))
+    assert(
+      array_equal(sorted(['5', '25', '125', '625'], 'auto'), [
+        '125',
+        '25',
+        '5',
+        '625',
+      ]),
+    )
+  })
+  await t.step('allequal function', () => {
+    const allequal = expressionFunctions.allequal
+    assert(allequal([1, 2, 1], [1, 2, 1]) === true)
+    assert(allequal([1, 2, 1], [1, 2, 3]) === false)
+    assert(allequal(['a', 'b', 'a'], ['a', 'b', 'a']) === true)
+    assert(allequal(['a', 'b', 'a'], ['a', 'b', 'c']) === false)
+    assert(allequal([1, 2, 1], [1, 2, 1, 2]) === false)
+    assert(allequal([1, 2, 1, 2], [1, 2, 1]) === false)
+  })
+})
diff --git a/bids-validator/src/schema/expressionLanguage.ts b/bids-validator/src/schema/expressionLanguage.ts
index f28f53bbd..aa6596a3f 100644
--- a/bids-validator/src/schema/expressionLanguage.ts
+++ b/bids-validator/src/schema/expressionLanguage.ts
@@ -83,9 +83,19 @@ export const expressionFunctions = {
     }
     return arg.substr(start, end - start)
   },
-  sorted: <T>(list: T[]): T[] => {
-    // Use a cmp function that will work for any comparable types
-    return list.toSorted((a, b) => +(a > b) - +(a < b))
+  sorted: <T>(list: T[], method: string = 'auto'): T[] => {
+    const cmp = {
+      numeric: (a: T, b: T) => {
+        return Number(a) - Number(b)
+      },
+      lexical: (a: T, b: T) => {
+        return String(a).localeCompare(String(b))
+      },
+      auto: (a: T, b: T) => {
+        return +(a > b) - +(a < b)
+      },
+    }[method]
+    return list.toSorted(cmp)
   },
   allequal: <T>(a: T[], b: T[]): boolean => {
     return (a != null && b != null) && a.length === b.length && a.every((v, i) => v === b[i])
diff --git a/bids-validator/src/schema/fixtures.test.ts b/bids-validator/src/schema/fixtures.test.ts
new file mode 100644
index 000000000..fea0f6aec
--- /dev/null
+++ b/bids-validator/src/schema/fixtures.test.ts
@@ -0,0 +1,97 @@
+import { FileTree } from '../types/filetree.ts'
+import { nullReadBytes } from '../tests/nullReadBytes.ts'
+
+const text = () => Promise.resolve('')
+
+const anatJson = () =>
+  Promise.resolve(
+    JSON.stringify({
+      rootOverwrite: 'anat',
+      subOverwrite: 'anat',
+      anatValue: 'anat',
+    }),
+  )
+const subjectJson = () =>
+  Promise.resolve(
+    JSON.stringify({ subOverwrite: 'subject', subValue: 'subject' }),
+  )
+const rootJson = () =>
+  Promise.resolve(JSON.stringify({ rootOverwrite: 'root', rootValue: 'root' }))
+
+export const rootFileTree = new FileTree('/', '')
+const stimuliFileTree = new FileTree('/stimuli', 'stimuli', rootFileTree)
+const subjectFileTree = new FileTree('/sub-01', 'sub-01', rootFileTree)
+const sessionFileTree = new FileTree(
+  '/sub-01/ses-01',
+  'ses-01',
+  subjectFileTree,
+)
+const anatFileTree = new FileTree(
+  '/sub-01/ses-01/anat',
+  'anat',
+  sessionFileTree,
+)
+
+export const dataFile = {
+  text,
+  path: '/sub-01/ses-01/anat/sub-01_ses-01_T1w.nii.gz',
+  name: 'sub-01_ses-01_T1w.nii.gz',
+  size: 311112,
+  ignored: false,
+  stream: new ReadableStream<Uint8Array>(),
+  readBytes: nullReadBytes,
+}
+
+anatFileTree.files = [
+  dataFile,
+  {
+    text: anatJson,
+    path: '/sub-01/ses-01/anat/sub-01_ses-01_T1w.json',
+    name: 'sub-01_ses-01_T1w.json',
+    size: 311112,
+    ignored: false,
+    stream: new ReadableStream<Uint8Array>(),
+    readBytes: nullReadBytes,
+  },
+]
+
+sessionFileTree.files = []
+sessionFileTree.directories = [anatFileTree]
+
+subjectFileTree.files = [
+  {
+    text: subjectJson,
+    path: '/sub-01/ses-01_T1w.json',
+    name: 'ses-01_T1w.json',
+    size: 311112,
+    ignored: false,
+    stream: new ReadableStream<Uint8Array>(),
+    readBytes: nullReadBytes,
+  },
+]
+subjectFileTree.directories = [sessionFileTree]
+
+stimuliFileTree.files = [...Array(10).keys()].map((i) => (
+  {
+    text,
+    path: `/stimuli/stimfile${i}.png`,
+    name: `stimfile${i}.png`,
+    size: 2048,
+    ignored: false,
+    stream: new ReadableStream<Uint8Array>(),
+    readBytes: nullReadBytes,
+  }
+))
+
+rootFileTree.files = [
+  {
+    text: rootJson,
+    path: '/T1w.json',
+    name: 'T1w.json',
+    size: 311112,
+    ignored: false,
+    stream: new ReadableStream<Uint8Array>(),
+    readBytes: nullReadBytes,
+  },
+]
+rootFileTree.directories = [stimuliFileTree, subjectFileTree]