Update the findRE and replaceRE functions (#1881)

Co-authored-by: anakinsleftleg <77084568+anakinsleftleg@users.noreply.github.com>

Closes #1793

2 files changed, 53 insertions, 44 deletions

diff --git a/content/en/functions/findRe.md b/content/en/functions/findRe.md
index b6375be9c..2896e3cd1 100644
--- a/content/en/functions/findRe.md
+++ b/content/en/functions/findRe.md
@@ -1,44 +1,40 @@
 ---
 title: findRE
-description: Returns a list of strings that match the regular expression.
-date: 2017-02-01
-publishdate: 2017-02-01
-lastmod: 2017-02-01
+description: Returns a slice of strings that match the regular expression.
 categories: [functions]
 menu:
   docs:
-    parent: "functions"
+    parent: functions
 keywords: [regex]
-signature: ["findRE PATTERN INPUT [LIMIT]"]
-workson: []
-hugoversion:
-relatedfuncs: []
-deprecated: false
+signature:
+  - "findRE PATTERN INPUT [LIMIT]"
+  - "strings.FindRE PATTERN INPUT [LIMIT]"
+relatedfuncs: [replaceRE]
 aliases: []
 ---
+By default, the `findRE` function finds all matches. You can limit the number of matches with an optional LIMIT paramater.
 
-By default all matches will be included. The number of matches can be limited with an optional third 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 example below returns a list of all second level headers (`<h2>`) in the content:
+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`.
 
-```
-{{ findRE "<h2.*?>(.|\n)*?</h2>" .Content }}
+This example returns a slice of all second level headings (`h2` elements) within the rendered `.Content`:
+
+```go-html-template
+{{ findRE `(?s)<h2.*?>.*?</h2>` .Content }}
 ```
 
-You can limit the number of matches in the list with a third parameter. The following example shows how to limit the returned value to just one match (or none, if there are no matched substrings):
+The `s` flag causes `.` to match `\n` as well, allowing us to find an `h2` element that contains newlines.
 
-```
-{{ findRE "<h2.*?>(.|\n)*?</h2>" .Content 1 }}
-    <!-- returns ["<h2 id="#foo">Foo</h2>"] -->
+To limit the number of matches to one:
+
+```go-html-template
+{{ findRE `(?s)<h2.*?>.*?</h2>` .Content 1 }}
 ```
 
 {{% note %}}
-Hugo uses Go's [Regular Expression package](https://golang.org/pkg/regexp/), which is the same general syntax used by Perl, Python, and other languages but with a few minor differences for those coming from a background in PCRE. For a full syntax listing, see the [GitHub wiki for re2](https://github.com/google/re2/wiki/Syntax).
-
-If you are just learning RegEx, or at least Go's flavor, you can practice pattern matching in the browser at <https://regex101.com/>.
+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 %}}
 
-[partials]: /templates/partials/
-[`plainify`]: /functions/plainify/
-[toc]: /content-management/toc/
-[`urlize`]: /functions/urlize
+[RE2]: https://github.com/google/re2/wiki/Syntax
+[string literal]: https://go.dev/ref/spec#String_literals
diff --git a/content/en/functions/replacere.md b/content/en/functions/replacere.md
index a2149b6d3..f1e34ba65 100644
--- a/content/en/functions/replacere.md
+++ b/content/en/functions/replacere.md
@@ -1,34 +1,47 @@
 ---
 title: replaceRE
-description: Replaces all occurrences of a regular expression with the replacement pattern.
-date: 2017-02-01
-publishdate: 2017-02-01
-lastmod: 2020-09-07
+description: Returns a string, replacing all occurrences of a regular expression with a replacement pattern.
 categories: [functions]
 menu:
   docs:
-    parent: "functions"
+    parent: functions
 keywords: [regex]
-signature: ["strings.ReplaceRE PATTERN REPLACEMENT INPUT [LIMIT]", "replaceRE PATTERN REPLACEMENT INPUT [LIMIT]"]
-workson: []
-hugoversion:
-relatedfuncs: []
-deprecated: false
+signature:
+  - "replaceRE PATTERN REPLACEMENT INPUT [LIMIT]"
+  - "strings.ReplaceRE PATTERN REPLACEMENT INPUT [LIMIT]"
+relatedfuncs: [findRE]
 aliases: []
 ---
+By default, the `replaceRE` function replaces all matches. You can limit the number of matches with an optional LIMIT paramater.
 
-`strings.ReplaceRE` returns a copy of `INPUT`, replacing all matches of the regular
-expression `PATTERN` with the replacement text `REPLACEMENT`.
-The number of replacements can be limited 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`.
+
+This example replaces two or more consecutive hyphens with a single hyphen:
+
+```go-html-template
+{{ $s := "a-b--c---d" }}
+{{ replaceRE `(-{2,})` "-" $s }} → a-b-c-d
 ```
-{{ replaceRE "^https?://([^/]+).*" "$1" "http://gohugo.io/docs" }}` → "gohugo.io"
-{{ "http://gohugo.io/docs" | replaceRE "^https?://([^/]+).*" "$1" }}` → "gohugo.io"
-{{ replaceRE "a+b" "X" "aabbaabbab" 1 }} → "Xbaabbab"
+
+To limit the number of replacements to one:
+
+```go-html-template
+{{ $s := "a-b--c---d" }}
+{{ replaceRE `(-{2,})` "-" $s 1 }} → a-b-c---d
 ```
 
-{{% note %}}
-Hugo uses Go's [Regular Expression package](https://golang.org/pkg/regexp/), which is the same general syntax used by Perl, Python, and other languages but with a few minor differences for those coming from a background in PCRE. For a full syntax listing, see the [GitHub wiki for re2](https://github.com/google/re2/wiki/Syntax).
+You can use `$1`, `$2`, etc. within the replacement string to insert the groups captured within the regular expression:
 
-If you are just learning RegEx, or at least Go's flavor, you can practice pattern matching in the browser at <https://regex101.com/>.
+```go-html-template
+{{ $s := "http://gohugo.io/docs" }}
+{{ replaceRE "^https?://([^/]+).*" "$1" $s }} → 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 %}}
+
+[RE2]: https://github.com/google/re2/wiki/Syntax
+[string literal]: https://go.dev/ref/spec#String_literals