空线和修剪

Let's combine anchors + the m flag for two extremely useful idioms: matching empty lines and trimming the whitespace at the start/end of each line.

Empty lines

An empty line is a position where ^ and $ coincide: zero characters between the start and the end of the line.

Pattern: ^$
Flag:    gm
Sample:  uno\n\ndue\n\n\ntre

The match is zero-width: the highlighter will show a marker between the two \n. To also match "empty but with spaces" lines, use ^\s*$.

Trimming a line

To remove whitespace at the start or end of a line, combine an anchor + the \s class + a quantifier:

^\s+    matches whitespace at the start of a line (can then be replaced with the empty string, module 7)
\s+$    matches whitespace at the end of a line

With the gm flags you collect every margin whitespace of every line.

Identifying empty lines and trailing spaces

A pure empty line is matched by ^$ (with the m flag). However, many apparently empty lines contain invisible spaces or tabs. The pattern ^\\s*$ intercepts these lines too, making data cleanup much easier.

Try it

Review exercise

Additional challenge