Filter column with awk and regexp

Question

I've a pretty simple question. I've a file containing several columns and I want to filter them using awk.

So the column of interest is the 6th column and I want to find every string containing :

• starting with a number from 1 to 100
• after that one "S" or a "M"
• again a number from 1 to 100
• after that one "S" or a "M"

So per example : 20S50M is ok

I tried :

awk '{ if($6 == '/[1-100][S|M][1-100][S|M]/') print} file.txt 

but it didn't work... What am I doing wrong?

Answer 1

This should do the trick:

awk '$6~/^(([1-9]|[1-9][0-9]|100)[SM]){2}$/' file 

Regexplanation:

^                        # Match the start of the string (([1-9]|[1-9][0-9]|100)  # Match a single digit 1-9 or double digit 10-99 or 100 [SM]                     # Character class matching the character S or M ){2}                     # Repeat everything in the parens twice $                        # Match the end of the string 

---

You have quite a few issue with your statement:

awk '{ if($6 == '/[1-100][S|M][1-100][S|M]/') print} file.txt 

• == is the string comparision operator. The regex comparision operator is ~.
• You don't quote regex strings (you never quote anything with single quotes in awk beside the script itself) and your script is missing the final (legal) single quote.
• [0-9] is the character class for the digit characters, it's not a numeric range. It means match against any character in the class 0,1,2,3,4,5,6,7,8,9 not any numerical value inside the range so [1-100] is not the regular expression for digits in the numerical range 1 - 100 it would match either a 1 or a 0.
• [SM] is equivalent to (S|M) what you tried [S|M] is the same as (S|\||M). You don't need the OR operator in a character class.

Awk using the following structure condition{action}. If the condition is True the actions in the following block {} get executed for the current record being read. The condition in my solution is $6~/^(([1-9]|[1-9][0-9]|100)[SM]){2}$/ which can be read as does the sixth column match the regular expression, if True the line gets printed because if you don't get any actions then awk will execute {print $0} by default.

Answer 2

Regexes cannot check for numeric values. "A number from 1 to 100" is outside what regexes can do. What you can do is check for "1-3 digits."

You want something like this

/\d{1,3}[SM]\d{1,3}[SM]/ 

Note that the character class [SM] doesn't have the ! alternation character. You would only need that if you were writing it as (S|M).