Clarify findRE, replaceRE, and findRESubmatch (#2064)

Closes #2063

content/en/functions/findRe.md

@@ -9,13 +9,25 @@ keywords: [regex]
 signature:
   - "findRE PATTERN INPUT [LIMIT]"
   - "strings.FindRE PATTERN INPUT [LIMIT]"
-relatedfuncs: [replaceRE]
+relatedfuncs: [findRESubmatch, replaceRE]
 ---
-By default, the `findRE` function finds all matches. You can limit the number of matches with an optional LIMIT parameter.
+By default, `findRE` finds all matches. You can limit the number of matches with an optional LIMIT parameter.
 
 When specifying the regular expression, use a raw [string literal] (backticks) instead of an interpreted string literal (double quotes) to simplify the syntax. With an interpreted string literal you must escape backslashes.
 
-The syntax of the regular expression is the same general syntax used by Perl, Python, and other languages. More precisely, it is the syntax accepted by [RE2] except for `\C`.
+[string literal]: https://go.dev/ref/spec#String_literals
+
+This function uses the [RE2] regular expression library. See the [RE2 syntax documentation] for details. Note that the RE2 `\C` escape sequence is not supported.
+
+[RE2]: https://github.com/google/re2/
+[RE2 syntax documentation]: https://github.com/google/re2/wiki/Syntax/
+
+{{% note %}}
+The RE2 syntax is a subset of that accepted by [PCRE], roughly speaking, and with various [caveats].
+
+[caveats]: https://swtch.com/~rsc/regexp/regexp3.html#caveats
+[PCRE]: https://www.pcre.org/
+{{% /note %}}
 
 This example returns a slice of all second level headings (`h2` elements) within the rendered `.Content`:
 
@@ -34,25 +46,3 @@ To limit the number of matches to one:
 {{% note %}}
 You can write and test your regular expression using [regex101.com](https://regex101.com/). Be sure to select the Go flavor before you begin.
 {{% /note %}}
-
-## findRESubmatch
-
-In Hugo 0.110.0 we added a variant of `findRE` that returns a slice of strings holding the text of the leftmost match of the regular expression in s and the matches, if any, of its subexpressions.
-
-This:
-
-```go-html-template
-{{ findRESubmatch §§<a\s*href="(.+?)">(.+?)</a>§§ §§<li><a href="#foo">Foo</a></li> <li><a href="#bar">Bar</a></li>§§ | print | safeHTML }}
-```
-
-Will print:
-
-```
-[[<a href=\"#foo\">Foo</a> #foo Foo] [<a href=\"#bar\">Bar</a> #bar Bar]]
-```
-
-{{< new-in "0.110.0" >}}
-
-
-[RE2]: https://github.com/google/re2/wiki/Syntax
-[string literal]: https://go.dev/ref/spec#String_literals

content/en/functions/findresubmatch.md

@@ -0,0 +1,102 @@
+---
+title: findRESubmatch
+description: Returns a slice of strings holding the text of the leftmost match of the regular expression and the matches, if any, of its subexpressions
+categories: [functions]
+menu:
+  docs:
+    parent: functions
+keywords: [regex]
+signature:
+  - "findRESubmatch PATTERN INPUT [LIMIT]"
+  - "strings.FindRESubmatch PATTERN INPUT [LIMIT]"
+relatedfuncs: [findRE, replaceRE]
+---
+
+By default, `findRESubmatch` finds all matches. You can limit the number of matches with an optional LIMIT parameter. A return value of nil indicates no match.
+
+When specifying the regular expression, use a raw [string literal] (backticks) instead of an interpreted string literal (double quotes) to simplify the syntax. With an interpreted string literal you must escape backslashes.
+
+[string literal]: https://go.dev/ref/spec#String_literals
+
+This function uses the [RE2] regular expression library. See the [RE2 syntax documentation] for details. Note that the RE2 `\C` escape sequence is not supported.
+
+[RE2]: https://github.com/google/re2/
+[RE2 syntax documentation]: https://github.com/google/re2/wiki/Syntax/
+
+{{% note %}}
+The RE2 syntax is a subset of that accepted by [PCRE], roughly speaking, and with various [caveats].
+
+[caveats]: https://swtch.com/~rsc/regexp/regexp3.html#caveats
+[PCRE]: https://www.pcre.org/
+{{% /note %}}
+
+## Demonstrative examples
+
+```go-html-template
+{{ findRESubmatch `a(x*)b` "-ab-" }} → [["ab" ""]]
+{{ findRESubmatch `a(x*)b` "-axxb-" }} → [["axxb" "xx"]]
+{{ findRESubmatch `a(x*)b` "-ab-axb-" }} → [["ab" ""] ["axb" "x"]]
+{{ findRESubmatch `a(x*)b` "-axxb-ab-" }} → [["axxb" "xx"] ["ab" ""]]
+{{ findRESubmatch `a(x*)b` "-axxb-ab-" 1 }} → [["axxb" "xx"]]
+```
+
+## Practical example
+
+This markdown:
+
+```text
+- [Example](https://example.org)
+- [Hugo](https://gohugo.io)
+```
+
+Produces this HTML:
+
+```html
+<ul>
+  <li><a href="https://example.org">Example</a></li>
+  <li><a href="https://gohugo.io">Hugo</a></li>
+</ul>
+```
+
+To match the anchor elements, capturing the link destination and text:
+
+```go-html-template
+{{ $regex := `<a\s*href="(.+?)">(.+?)</a>` }}
+{{ $matches := findRESubmatch $regex .Content }}
+```
+
+Viewed as JSON, the data structure of `$matches` in the code above is:
+
+```json
+[
+  [
+    "<a href=\"https://example.org\"></a>Example</a>",
+    "https://example.org",
+    "Example"
+  ],
+  [
+    "<a href=\"https://gohugo.io\">Hugo</a>",
+    "https://gohugo.io",
+    "Hugo"
+  ]
+]
+```
+
+To render the `href` attributes:
+
+```go-html-template
+{{ range $matches }}
+  {{ index . 1 }}
+{{ end }}
+```
+
+Result:
+
+```text
+https://example.org
+https://gohugo.io
+```
+
+{{% note %}}
+You can write and test your regular expression using [regex101.com](https://regex101.com/). Be sure to select the Go flavor before you begin.
+{{% /note %}}

content/en/functions/replacere.md

@@ -5,17 +5,29 @@ categories: [functions]
 menu:
   docs:
     parent: functions
-keywords: [replace regex]
+keywords: [regex]
 signature:
   - "replaceRE PATTERN REPLACEMENT INPUT [LIMIT]"
   - "strings.ReplaceRE PATTERN REPLACEMENT INPUT [LIMIT]"
-relatedfuncs: [replace,findRE]
+relatedfuncs: [findRE, FindRESubmatch, replace]
 ---
-By default, the `replaceRE` function replaces all matches. You can limit the number of matches with an optional LIMIT parameter.
+By default, `replaceRE` replaces all matches. You can limit the number of matches with an optional LIMIT parameter.
 
 When specifying the regular expression, use a raw [string literal] (backticks) instead of an interpreted string literal (double quotes) to simplify the syntax. With an interpreted string literal you must escape backslashes.
 
-The syntax of the regular expression is the same general syntax used by Perl, Python, and other languages. More precisely, it is the syntax accepted by [RE2] except for `\C`.
+[string literal]: https://go.dev/ref/spec#String_literals
+
+This function uses the [RE2] regular expression library. See the [RE2 syntax documentation] for details. Note that the RE2 `\C` escape sequence is not supported.
+
+[RE2]: https://github.com/google/re2/
+[RE2 syntax documentation]: https://github.com/google/re2/wiki/Syntax/
+
+{{% note %}}
+The RE2 syntax is a subset of that accepted by [PCRE], roughly speaking, and with various [caveats].
+
+[caveats]: https://swtch.com/~rsc/regexp/regexp3.html#caveats
+[PCRE]: https://www.pcre.org/
+{{% /note %}}
 
 This example replaces two or more consecutive hyphens with a single hyphen: