How to Ignore Optional Whitespace in Regular Expressions for HTML Parsing?

Optional Whitespace in Regular Expressions

When parsing HTML or text data, ignoring whitespace between certain characters is often necessary. However, this can be challenging using regular expressions.

Solution Using s? and s* Quantifiers

To match optional whitespace between characters, use the quantifiers s? and s*.

• s matches any whitespace character (space, tab, newline, etc.).
• ? means the preceding character may occur once or not at all.
• * means the preceding character may occur zero or more times.

Example

To ignore whitespace in the following HTML tags:

<code class="html"><a href="/wiki/File:Sky1.png" title="File:Sky1.png">
<img alt="Sky1.png" src="http://media-mcw.cursecdn.com/thumb/5/56/Sky1.png/150px-Sky1.png" width="150" height="84">
</a></code>

Use the following regular expression:

'#<a href\s?="(.*?)" title\s?="(.*?)"><img alt\s?="(.*?)" src\s?="(.*?)"[\s*]width\s?="150"[\s*]height\s?="(.*?)"></a>#'

This expression allows for optional whitespace between the attribute names and their values, as well as between the attribute values and the surrounding HTML tags.

Note on Character Classes

The original code used the character class [s], which caused unexpected results. A character class matches any of its members once, and the quantifier allows it to occur multiple times. By replacing [s] with s, you ensure that only whitespace characters are matched and that the quantifier applies to them specifically.

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