Sure, here is a regular expression to match an attribute in an HTML code:

<\w+ \w+=\"([^\"]+)\">

The < metacharacter matches the beginning of an HTML tag. The \w+ metacharacter matches one or more word characters. The \w+ metacharacter matches one or more word characters. The =\" metacharacter matches an equal sign. The ([^\"]+) part of the pattern matches any character except a double quote, and the \"> metacharacter matches the end of an HTML tag.

So, this regex pattern will match any HTML tag that has an attribute with a value.

For example, the following HTML code would be matched by this regex pattern:

HTML

<input type="text" name="username">

The type and name attributes would be matched by the pattern.

The following HTML code would not be matched by this regex pattern:

HTML

<h1>This is a heading</h1>

The h1 tag does not have any attributes.