Why is this Regex not capturing both matching lines?

Question

I have the following text file:

a
a

I am trying to match lines that start with a using the following command: pcregrep -M '^a'. It is matching only the first a and not the second. Does anyone know why? I am using pcregrep because this is a simple problem that I'm expanding to more complex scenarios later.

Thanks!

UPDATE

The reason is that I'm using Mac OS, in which every newline is a carriage return. Because of this, pcregrep interprets the file contents (which is a\ra) as one line, and my regex returns just the first a from that line since that's all I've specified in the expression. The solution to this, with pcregrep, is to specify the newline type. 'Newline type' means the character that the Regex engine interprets as designating the end of a line. Thus, if we specify in this case that the newline type is carriage return (\r), pcregrep will interpret my file's contents as two lines, and will match and return both.

The fixed version of my Regex is pcregrep -M -N CR '^a', where -N CR means "newline type is carriage return".

are you sure the linebreak is the correct linebreak for the system you are running grep on?... CR-LF if it is on a PC? or did you pull the data from a Linux or Mac system without converting the linebreaks? — , Jun 04 '19 at 20:23
["Carriage return"](https://en.wikipedia.org/wiki/Carriage_return), `\x0D` or `\r` char. — Wiktor Stribiżew, Jun 04 '19 at 20:27
@WiktorStribiżew I'm not sure...I did the line break just by hitting Enter. VsCode on OS X. Why does it make a difference? — gkeenley, Jun 04 '19 at 20:29
That is it: *Mac OSX*. Default line breaks are CR only there. — Wiktor Stribiżew, Jun 04 '19 at 20:29
I'm looking at the docs and I'm not sure I understand what the -N option actually does. So OS X interprets any newline as a carriage return, right? If so, what does -N do? I tried using it with all the different values ("-N ANY" etc) and nothing works so far... — gkeenley, Jun 04 '19 at 20:35
Ah scratch that, "-N CR" actually does work. Thanks! Do you want to write that as an answer so I can approve? — gkeenley, Jun 04 '19 at 20:39
@WiktorStribiżew I updated my initial post. Let me know if you think I got anything wrong there. — gkeenley, Jun 04 '19 at 20:48

score 1 · Answer 1 · answered Jun 08 '19 at 20:57

Since you created a text file in MasOS, the line endings are represented with CR (carriage return) symbol (\r, \x0D, a char with Index 13 in the ASCII table).

By default, pcregrep and other suchlike tools consider \n, an LF (line feed) symbol as the line break char.

You should tell pcregrep to use CR as line break chars using -N option:

pcregrep -o -N CR '^a' file
            ^^^^^

Why is this Regex not capturing both matching lines?

1 Answers1