Update HTML input pattern attribute documentation (fixes #26907)

I've tried to apply the feedback from #29842 and reorganized some of the sections to hopefully make them clearer.

files/en-us/web/html/attributes/pattern/index.md

@@ -15,25 +15,30 @@ The **`pattern`** attribute specifies a [regular expression](/en-US/docs/Web/Jav
 
 The `pattern` attribute is an attribute of the {{HTMLElement("input/text", "text")}}, {{HTMLElement("input/tel", "tel")}}, {{HTMLElement("input/email", "email")}}, {{HTMLElement("input/url", "url")}}, {{HTMLElement("input/password", "password")}}, and {{HTMLElement("input/search", "search")}} input types.
 
-The `pattern` attribute, when specified, is a regular expression which the input's [`value`](/en-US/docs/Web/HTML/Element/input#value) must match for the value to pass [constraint validation](/en-US/docs/Web/HTML/Constraint_validation). It must be a valid JavaScript regular expression, as used by the {{jsxref("RegExp")}} type, and as documented in our [guide on regular expressions](/en-US/docs/Web/JavaScript/Guide/Regular_expressions); the `'u'` flag is specified when compiling the regular expression so that the pattern is treated as a sequence of Unicode code points, instead of as ASCII. No forward slashes should be specified around the pattern text.
+The `pattern` attribute, when specified, is a regular expression which the input's [`value`](/en-US/docs/Web/HTML/Element/input#value) must match for the value to pass [constraint validation](/en-US/docs/Web/HTML/Constraint_validation). It must be a valid JavaScript regular expression, as used by the {{jsxref("RegExp")}} type, and as documented in our [guide on regular expressions](/en-US/docs/Web/JavaScript/Guide/Regular_expressions).
 
-If the specified pattern is not specified or is invalid, no regular expression is applied and this attribute is ignored.
+The pattern's regular expression is compiled with the [`'v'` flag](/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Character_class#v-mode_character_class). This makes the regular expression [unicode-aware](/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/unicode#unicode-aware_mode), and also changes how character classes are interpreted. This allows character class set intersection and subtraction operations, and in addition to `]` and `\`, the following characters must be escaped using a `\` backslash if they represent literal characters: `(`, `)`, `[`, `{`, `}`, `/`, `-`, `|`. Before mid-2023, the `'u'` flag was specified instead; If you're updating older code, [this document outlines the differences](https://github.com/tc39/proposal-regexp-v-flag#how-is-the-v-flag-different-from-the-u-flag).
 
-> **Note:** Use the [`title`](/en-US/docs/Web/HTML/Element/input#title) attribute to specify text that most browsers will display as a tooltip to explain what the requirements are to match the pattern. You **must not** rely on the tooltip alone for an explanation. See below for more information on usability.
+The pattern's regular expression must match the entire input's `value`, rather than matching a substring - as if a `^(?:` were implied at the start of the pattern and `)$` at the end.
 
-Some of the input types supporting the pattern attribute, notably the {{HTMLElement("input/email", "email")}} and {{HTMLElement("input/url", "url")}} input types, have expected value syntaxes that must be matched. If the pattern attribute isn't present, and the value doesn't match the expected syntax for that value type, the {{domxref('ValidityState')}} object's read-only {{domxref('ValidityState.typeMismatch','typeMismatch')}} property will be true.
-
-### Usability
+No forward slashes should be specified around the pattern text. If the attribute value is not present, the empty string, or invalid, no regular expression is applied.
 
-When including a `pattern`, provide a description of the pattern in visible text near the control. Additionally, include a [`title`](/en-US/docs/Web/HTML/Global_attributes/title) attribute which gives a description of the pattern. User agents may use the title contents during constraint validation to tell the user that the pattern is not matched. Some browsers show a tooltip with title contents, improving usability for sighted users. Additionally, assistive technology may read the title aloud when the control gains focus, but this should not be relied upon for accessibility.
+Some of the input types supporting the pattern attribute, notably the {{HTMLElement("input/email", "email")}} and {{HTMLElement("input/url", "url")}} input types, have expected value syntaxes that must be matched. If the pattern attribute isn't present, and the value doesn't match the expected syntax for that value type, the {{domxref('ValidityState')}} object's read-only {{domxref('ValidityState.typeMismatch','typeMismatch')}} property will be true.
 
 ### Constraint validation
 
 If the input's value is not the empty string and the value does not match the entire regular expression, there is a constraint violation reported by the {{domxref('ValidityState')}} object's {{domxref('ValidityState.patternMismatch','patternMismatch')}} property being `true`.
-The pattern's regular expression, when matched against the value, must have its start anchored to the start of the string and its end anchored to the end of the string, which is slightly different from JavaScript regular expressions: in the case of pattern attribute, we are matching against the entire value, not just any subset, as if a `^(?:` were implied at the start of the pattern and `)$` at the end.
 
 > **Note:** If the `pattern` attribute is specified with no value, its value is implicitly the empty string. Thus, **any non-empty** input `value` will result in constraint violation.
 
+### Usability and accessibility considerations
+
+When including a `pattern`, provide a description of the pattern in visible text near the control. Additionally, include a [`title`](/en-US/docs/Web/HTML/Element/input#title) attribute which gives a description of the pattern. User agents may use the title contents during constraint validation to tell the user that the pattern is not matched. Some browsers show a tooltip with title contents, improving usability for sighted users. Additionally, assistive technology may read the title aloud when the control gains focus, but this should not be relied upon for accessibility.
+
+Only relying on the `title` attribute for the visual display of text content is discouraged, as many user agents do not expose the attribute in an accessible manner. Although some browsers show a tooltip when an element with a title is hovered, that leaves out keyboard-only and touch-only users. This is one of the several reasons you must include information informing users how to fill out the control to match the requirements.
+
+While `title`s are used by some browsers to populate error messaging, because browsers sometimes also show the title as text on hover, it therefore shows in non-error situations, so be careful not to word titles as if an error has occurred.
+
 ## Examples
 
 ### Matching a phone number
@@ -142,12 +147,6 @@ This renders like so:
 
 {{ EmbedLiveSample('Specifying_a_pattern', 600, 110) }}
 
-### Accessibility Concerns
-
-When a control has a `pattern` attribute, the `title` attribute, if used, must describe the pattern. Relying on the `title` attribute for the visual display of text content is generally discouraged as many user agents do not expose the attribute in an accessible manner. Some browsers show a tooltip when an element with a title is hovered, but that leaves out keyboard-only and touch-only users. This is one of the several reasons you must include information informing users how to fill out the control to match the requirements.
-
-While `title`s are used by some browsers to populate error messaging, because browsers sometimes also show the title as text on hover, it therefore shows in non-error situations, so be careful not to word titles as if an error has occurred.
-
 ## Specifications
 
 {{Specifications}}