fix: make the meaning of ^ and $ more precise (#1905)

* make the meaning of ^ and $ more clear

^ and $ work differently in multi-line pattern which are different from \A and \Z.

* Update regexp.md

Co-authored-by: Rico Sta. Cruz <rstacruz@users.noreply.github.com>

regexp.md

@@ -36,10 +36,10 @@ description: |
 ### Anchors
 
 | Pattern | Description            |
-| ------- | ----------------------- |
+| ------- | ---------------------- |
 | `\G`    | Start of match         |
-| `^`     | Start of string         |
-| `$`     | End of string           |
+| `^`     | Start of string \*     |
+| `$`     | End of string \*       |
 | `\A`    | Start of string        |
 | `\Z`    | End of string          |
 | `\z`    | Absolute end of string |
@@ -48,6 +48,8 @@ description: |
 | `^abc`  | Start with `abc`       |
 | `abc$`  | End with `abc`         |
 
+For multiline patterns (`m` flag), `^` and `$` will act as start and end of line.
+
 ### Escaped characters
 
 | Pattern    | Description                            |