plugin: search and replace

internal/plugin/searchreplace/README.md

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+# Search and Replace Plugin
+
+This plugin allows users to perform search and replace operations on messages by replying to a message with a search/replace command.
+
+## Usage
+
+To use the plugin, reply to any message with a command in the following format:
+
+```
+s/search/replace/[flags]
+```
+
+Where:
+- `search` is the text you want to find (case-sensitive by default)
+- `replace` is the text you want to substitute in place of the search term
+- `flags` (optional) control the behavior of the replacement
+
+### Supported Flags
+
+- `g` - Global: Replace all occurrences of the search term (without this flag, only the first occurrence is replaced)
+- `i` - Case insensitive: Match regardless of case
+- `n` - Treat search pattern as a regular expression (advanced users)
+
+### Examples
+
+1. Basic replacement (replaces first occurrence):
+   ```
+   s/hello/hi/
+   ```
+
+2. Global replacement (replaces all occurrences):
+   ```
+   s/hello/hi/g
+   ```
+
+3. Case-insensitive replacement:
+   ```
+   s/Hello/hi/i
+   ```
+
+4. Combined flags (global and case-insensitive):
+   ```
+   s/hello/hi/gi
+   ```
+
+## Limitations
+
+- The plugin can only access the text content of the original message
+- Regular expression support is available with the `n` flag, but should be used carefully as invalid regex patterns will cause errors
+- The plugin does not modify the original message; it creates a new message with the replaced text