awk regex start of line anchor matches whitespace

Question

Parsing an input file through awk I ran into an issue with anchors in awk.

Given the following file:

 2015
2015
test
 test

Output with awk

$ awk '$1 ~ /^[0-9]/' file
 2015
2015

Output with sed

$ sed -n '/^[0-9]/p' file
2015

Can somebody explain the behaviour I'm seeing in awk?

Seen with

• CentOS 7, GNU bash 4.2.46, GNU Awk 4.0.2
• AIX 7, GNU bash 4.3.30, awk (default version in AIX), and gawk 4.0.2

Answer 1

You will understand the difference with this awk command:

awk '/^[0-9]/' file
2015

Now awk is operating on full line like sed not just the first field.

$1 ~ /^[0-9]/ only compares first field and since whitespace is default field separator in awk therefore first field is 2015 in both the lines irrespective of spaces before it.

Answer 2

The problem is you are picking the first field.

You should be doing awk '/^[0-9]/' file which matches the whole line.

To be more precise:

awk '$0 ~ /^[0-9]/' file

Is what you want, as $0 is the whole line.