Add regex cheat-sheet icon next to regex inputs

Adds RegexHelpIcon, a hover/click popover (modeled on HelpIcon) showing
a compact cheat sheet for the Rust `regex` crate's actual syntax, and
places it next to every user-facing regex field: Params/Metrics message
match regex, Alerts field regex, and FilterBar's regex(...) helper.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/Alerts.tsx

@@ -11,6 +11,7 @@ import {
 } from '../lib/api'
 import { sectionHeaderClassName, sectionHeaderLeftClassName } from './CollapsibleSection'
 import { HelpIcon } from './HelpIcon'
+import { RegexHelpIcon } from './RegexHelpIcon'
 import { queryInputClassName } from './FilterBar'
 
 type ConditionKind = AlertCondition['kind']
@@ -323,7 +324,10 @@ export function Alerts() {
           {form.kind === 'newvalue' && (
             <div className="space-y-1">
               <div>
-                <label className="mb-1 block text-xs text-text-subtle">Field regex</label>
+                <label className="mb-1 flex items-center gap-1 text-xs text-text-subtle">
+                  Field regex
+                  <RegexHelpIcon label="Help: Field regex syntax" />
+                </label>
                 <input
                   className="w-full rounded border border-border-soft bg-surface-1 px-2 py-1 font-mono text-sm text-text placeholder:text-text-subtle focus:border-border-strong focus:outline-none"
                   value={form.fieldRegex}

webapp/src/components/FilterBar.tsx

@@ -1,6 +1,7 @@
 import type { ReactNode } from 'react'
 import { useState } from 'react'
 import { HelpIcon } from './HelpIcon'
+import { RegexHelpIcon } from './RegexHelpIcon'
 
 // Reused by `Alerts.tsx`'s condition editor so alert conditions look like the filter bar without duplicating its draft/submit state.
 export const queryInputClassName =
@@ -30,6 +31,7 @@ export function FilterBar({
     >
       <div className="flex items-center gap-2">
         <HelpIcon sectionId="filtering" label="Help: Filtering" />
+        <RegexHelpIcon label="Help: regex(...) syntax" />
         {collapseToggle}
         <input
           className={queryInputClassName}

webapp/src/components/Metrics.tsx

@@ -12,6 +12,7 @@ import {
 } from '../lib/api'
 import { CollapseToggle, PopOutButton, sectionHeaderClassName, sectionHeaderLeftClassName, useSectionCollapsed } from './CollapsibleSection'
 import { HelpIcon } from './HelpIcon'
+import { RegexHelpIcon } from './RegexHelpIcon'
 
 const EMPTY_MATCHER: MetricParamMatcher = { param_name: '', capture_group: '', kind: 'string' }
 
@@ -174,7 +175,10 @@ export function Metrics({ range }: { range: TimeRange | null }) {
             />
           </div>
           <div>
-            <label className="mb-1 block text-xs text-text-subtle">Message match regex</label>
+            <label className="mb-1 flex items-center gap-1 text-xs text-text-subtle">
+              Message match regex
+              <RegexHelpIcon label="Help: Message match regex syntax" />
+            </label>
             <input
               className="w-full rounded border border-border-soft bg-surface-1 px-2 py-1 font-mono text-sm text-text placeholder:text-text-subtle focus:border-border-strong focus:outline-none"
               value={regex}

webapp/src/components/Params.tsx

@@ -2,6 +2,7 @@ import { useState } from 'react'
 import { ApiError, declareParam, type ParamDeclaration } from '../lib/api'
 import { CollapseToggle, PopOutButton, sectionHeaderClassName, sectionHeaderLeftClassName, useSectionCollapsed } from './CollapsibleSection'
 import { HelpIcon } from './HelpIcon'
+import { RegexHelpIcon } from './RegexHelpIcon'
 
 const EMPTY_KIND: ParamDeclaration['kind'] = 'string'
 
@@ -57,7 +58,10 @@ export function Params({ params, onDeclared }: { params: ParamDeclaration[]; onD
             />
           </div>
           <div>
-            <label className="mb-1 block text-xs text-text-subtle">Message match regex</label>
+            <label className="mb-1 flex items-center gap-1 text-xs text-text-subtle">
+              Message match regex
+              <RegexHelpIcon label="Help: Message match regex syntax" />
+            </label>
             <input
               className="w-full rounded border border-border-soft bg-surface-1 px-2 py-1 font-mono text-sm text-text placeholder:text-text-subtle focus:border-border-strong focus:outline-none"
               value={regex}

webapp/src/components/RegexHelpIcon.tsx

@@ -0,0 +1,67 @@
+import { useEffect, useRef, useState } from 'react'
+
+// Regex syntax cheat sheet, scoped to what the Rust `regex` crate actually supports (used server-side
+// for message-match/field-match regexes and the filter bar's `regex(...)`) — no PCRE-only lookaround
+// or backreferences, since those would silently fail to compile there.
+const CHEAT_SHEET: { tokens: string; meaning: string }[] = [
+  { tokens: '.', meaning: 'Any character except newline' },
+  { tokens: '\\d  \\w  \\s', meaning: 'Digit, word char, whitespace' },
+  { tokens: '\\D  \\W  \\S', meaning: 'Negated versions of the above' },
+  { tokens: '*  +  ?', meaning: '0+, 1+, 0-1 of the previous item' },
+  { tokens: '{n}  {n,}  {n,m}', meaning: 'Exactly n, n-or-more, n-to-m repeats' },
+  { tokens: '^  $', meaning: 'Start / end of text (or line, with (?m))' },
+  { tokens: '\\b  \\B', meaning: 'Word boundary / non-boundary' },
+  { tokens: '[abc]  [^abc]', meaning: 'Character class / negated class' },
+  { tokens: '[a-z0-9]', meaning: 'Character range' },
+  { tokens: 'a|b', meaning: 'Alternation' },
+  { tokens: '(...)', meaning: 'Capturing group' },
+  { tokens: '(?P<name>...)', meaning: 'Named capturing group' },
+  { tokens: '(?:...)', meaning: 'Non-capturing group' },
+  { tokens: '(?i)  (?m)  (?s)', meaning: 'Inline flags: case-insensitive, multi-line, dot-matches-newline' },
+]
+
+// "@" icon next to regex inputs; shows a compact cheat sheet for the regex flavor the server actually
+// uses (Rust `regex` crate — no lookaround/backreferences), modeled on HelpIcon's button styling.
+export function RegexHelpIcon({ label }: { label?: string }) {
+  const [open, setOpen] = useState(false)
+  const ref = useRef<HTMLDivElement>(null)
+
+  useEffect(() => {
+    if (!open) return
+    function onClickOutside(e: MouseEvent) {
+      if (ref.current && !ref.current.contains(e.target as Node)) setOpen(false)
+    }
+    document.addEventListener('mousedown', onClickOutside)
+    return () => document.removeEventListener('mousedown', onClickOutside)
+  }, [open])
+
+  return (
+    <div ref={ref} className="relative inline-flex" onMouseEnter={() => setOpen(true)} onMouseLeave={() => setOpen(false)}>
+      <button
+        type="button"
+        onClick={() => setOpen((v) => !v)}
+        title="Regex syntax cheat sheet"
+        aria-label={label ?? 'Regex syntax cheat sheet'}
+        className="flex h-5 w-5 shrink-0 items-center justify-center rounded-full border border-border-soft bg-surface-2 text-xs leading-none text-text-muted hover:border-border-strong hover:text-text-secondary"
+      >
+        @
+      </button>
+      {open && (
+        <div className="absolute left-0 top-full z-10 mt-1 w-72 rounded border border-border-soft bg-surface-0 p-2 shadow-lg">
+          <p className="mb-1.5 text-[11px] font-medium text-text-secondary">Regex syntax (Rust regex crate)</p>
+          <table className="w-full border-collapse text-[11px]">
+            <tbody>
+              {CHEAT_SHEET.map(({ tokens, meaning }) => (
+                <tr key={tokens}>
+                  <td className="whitespace-nowrap py-0.5 pr-2 align-top font-mono text-text">{tokens}</td>
+                  <td className="py-0.5 align-top text-text-muted">{meaning}</td>
+                </tr>
+              ))}
+            </tbody>
+          </table>
+          <p className="mt-1.5 text-[10px] text-text-subtle">No lookaround or backreferences (not supported by the Rust regex crate).</p>
+        </div>
+      )}
+    </div>
+  )
+}