Practical Vim, 2nd Edition

Tip 76

Use Parentheses to Capture Submatches

When specifying a pattern, we can capture submatches and then reference them elsewhere. This feature is especially useful in combination with the substitute command, but it can also be used to define patterns where a word is repeated.

Take this excerpt of text:

patterns/springtime.txt

I love Paris in the
the springtime.

Can you spot the grammatical error? It’s surprisingly hard to see because of a trick that our mind plays on us, but it should pop out with emphasis: “Paris in the the springtime.” When a line break separates two occurrences of the same word, our brain tends to ignore the duplicate. The effect is called a lexical illusion.^[20]

Here’s a regular expression that matches duplicate ...