docs: add docs

Author: mdjastrzebski

website/docs/api/constructs.md

@@ -9,7 +9,7 @@ These functions and objects represent available regex constructs.
 
 ```ts
 function choiceOf(
-  ...alternatives: RegexSequence[]
+  ...alternatives: RegexSequence[],
 ): ChoiceOf {
 ```
 
@@ -22,7 +22,9 @@ Example: `choiceOf("color", "colour")` matches either `color` or `colour` patter
 ### `capture()`
 
 ```ts
-function capture(sequence: RegexSequence): Capture;
+function capture(
+  sequence: RegexSequence,
+): Capture;
 ```
 
 Regex syntax: `(...)`.
@@ -35,3 +37,34 @@ TS Regex Builder does not have a construct for non-capturing groups. Such groups
 
 :::
 
+### `regex()`
+
+```ts
+function regex(
+  sequence: RegexSequence,
+): Regex;
+```
+
+Regex syntax: the pattern remains unchanged when wrapped by this construct.
+
+This construct is a no-op operator that groups array of `RegexElements` into a single element for composition purposes. This is particularly useful for defining smaller sequence patterns as separate variables.
+
+Without `regex()`:
+
+```ts
+const exponent = [anyOf('eE'), optional(anyOf('+-')), oneOrMore(digit)];
+const numberWithExponent = buildRegExp([
+  oneOrMore(digit),
+  ...exponent, // Need to spread "exponent" as it's an array.
+]);
+```
+
+With `regex()`:
+
+```ts
+const exponent = regex([anyOf('eE'), optional(anyOf('+-')), oneOrMore(digit)]);
+const numberWithExponent = buildRegExp([
+  oneOrMore(digit),
+  exponent, // Easily compose "exponent" sequence as a single element.
+]);
+```