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copied!<p>Regular expressions are a method of describing regular grammars. They accomplish this in a very concise and very efficient way. This makes them look complicated.</p> <p>They are also structured and decodeable.</p> <p>First, there is a <code>sed</code> call.</p> <pre><code>sed "{operation}/{expression}/{replacement}/{modifiers}" {argument} </code></pre> <p>Note </p> <ul> <li>that <code>sed</code> separates the parts with forward slashes. This means you cannot have an unescaped forward slash in <code>{expression}</code> or <code>{replacement}</code>.</li> <li>that unlike most other regex dialets, <code>sed</code> uses parentheses to match actual parentheses and <em>escaped</em> parentheses to define capturing groups.</li> </ul> <p>The <code>{operation}</code> happens to be <code>s</code> - substitute. </p> <p>The <code>{expression}</code> is <code>\(^[a-z,0-9]\)\(.*\)\( [a-z,0-9]*$\)</code>, which breaks down as</p> <pre> \( # start capture group 1 ^ # match the start of the string [a-z,0-9] # match characters a-z and 0-9 and a comma (!) \) # end capture group 1 \( # start capture group 2 .* # match any character (.), zero or more times (*) \) # end capture group 2 \( # start capture group 3 # match a space [a-z,0-9]* # match characters a-z and 0-9 and a comma (!) $ # match the end of the string \) # end capture group 3 </pre> <p>Just think for a second, how much code (and time) it would take you to write a function that does the same, and how little space a regex needs. That's why it's harder to read - it's extremely compressed.</p> <p>The <code>{replacement}</code> is <code>\1\2 \1</code>. <code>\n</code> is called a back-reference, where <code>n</code> is the number ofthe capturing group. So this inserts the contents of groups 1 and 2, a space and the contents of group 1 again.</p> <p>The <code>{modifiers}</code> part is a <code>g</code> flag, that makes the regex apply as often as possible. In this special case it doesn't make much sense since the regex above can only ever match once anyway.</p>

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