docs(regex): add regex flavor information (#26797)

lib/modules/manager/custom/regex/readme.md

@@ -1,5 +1,9 @@
 With the `regex` manager you can configure Renovate so it finds dependencies that are not detected by its other built-in package managers.
 
+Renovate supports the `ECMAScript (JavaScript)` flavor of regex.
+
+Renovate uses the `uhop/node-re2` package that provides bindings for [`google/re2`](https://github.com/google/re2).
+Read about [`uhop/node-re2`'s limitations in their readme](https://github.com/uhop/node-re2#limitations-things-re2-does-not-support).
 The `regex` manager is unique in Renovate because:
 
 - It is configurable via regex named capture groups
@@ -29,11 +33,11 @@ Configuration-wise, it works like this:
 - You can optionally have a `packageName` capture group or a `packageNameTemplate` if it differs from `depName`
 - You must have either a `datasource` capture group or a `datasourceTemplate` config field
 - You can optionally have a `depType` capture group or a `depTypeTemplate` config field
-- You can optionally have a `versioning` capture group or a `versioningTemplate` config field. If neither are present, `semver-coerced` will be used as the default
+- You can optionally have a `versioning` capture group or a `versioningTemplate` config field. If neither are present, Renovate will use `semver-coerced` as the default
 - You can optionally have an `extractVersion` capture group or an `extractVersionTemplate` config field
-- You can optionally have a `currentDigest` capture group.
+- You can optionally have a `currentDigest` capture group
-- You can optionally have a `registryUrl` capture group or a `registryUrlTemplate` config field. If it's a valid URL, it will be converted to the `registryUrls` field as a single-length array.
+- You can optionally have a `registryUrl` capture group or a `registryUrlTemplate` config field. If it's a valid URL, it will be converted to the `registryUrls` field as a single-length array
-- You can optionally have an `indentation` capture group. It must be either empty or whitespace only, otherwise it will be reset to an empty string.
+- You can optionally have an `indentation` capture group. It must be either empty or whitespace only, otherwise it will be reset to an empty string
 
 ### Regular Expression Capture Groups
 
@@ -51,6 +55,7 @@ RUN curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version ${YARN_VER
 You would need to capture the `currentValue` with a named capture group, like this: `ENV YARN_VERSION=(?<currentValue>.*?)\\n`.
 
 If you're looking for an online regex testing tool that supports capture groups, try [regex101.com](<https://regex101.com/?flavor=javascript&flags=g&regex=ENV%20YARN_VERSION%3D(%3F%3CcurrentValue%3E.*%3F)%5Cn&testString=FROM%20node%3A12%0AENV%20YARN_VERSION%3D1.19.1%0ARUN%20curl%20-o-%20-L%20https%3A%2F%2Fyarnpkg.com%2Finstall.sh%20%7C%20bash%20-s%20--%20--version%20%24%7BYARN_VERSION%7D>).
+You must select the `ECMAScript (JavaScript)` flavor of regex.
 Be aware that backslashes (`'\'`) of the resulting regex have to still be escaped e.g. `\n\s` --> `\\n\\s`.
 You can use the Code Generator in the sidebar and copy the regex in the generated "Alternative syntax" comment into JSON.
 
@@ -121,7 +126,7 @@ You could configure Renovate to update the `Dockerfile` like this:
 }
 ```
 
-We could drop the `versioningTemplate` because Renovate defaults to `∆semver-coerced` versioning.
+We could drop the `versioningTemplate` because Renovate defaults to `semver-coerced` versioning.
 But we included the `versioningTemplate` config option to show you why we call these fields _templates_: because they are compiled using Handlebars and so can be composed from values you collect in named capture groups.
 
 You should use triple brace `{{{ }}}` templates like `{{{versioning}}}` to be safe.