What does the regular expression ^^^ mean?

Question

I am trying to understand the following Basic Regular Expression pattern:

^^^

This is how I interpret it as:

The first caret character is treated as the anchor for marking the beginning of line.
The second and third caret characters are matched literally (as they are neither inside a character class to cause negation, and nor the first character to be treated as anchor).

So, as I understand, I expect it to match the occurrences of ^^ pattern at the beginning of a line. Do I understand it right?

However, it appears that the aforementioned regular expression matches every line in the file irrespective of its contents. This behaviour is observed when searching a text file in Sublime Text with Regex search mode enabled.

What am I missing? How do I correctly interpret the regular expression?

Update: I observe a different behavior when running the following command using bash shell:

grep "^^^" foo

where foo is the same text file as above. The regular expression matches all the lines containing the pattern ^^ towards the beginning of line.

I am now confused as to why the result differs in two cases, and how do I understand the behavior fully?

They're all separate tokens. It's equivalent to `^`. It matches the position at the beginning of the string, then it matches the position at the beginning of the string, then it matches the position at the beginning of the string. — CertainPerformance, Mar 02 '20 at 01:47
@CertainPerformance Right, thats exactly what appears to be the case. I am having a hard time interpreting multiple occurrences of the caret character. — Nimesh Neema, Mar 02 '20 at 01:49
@CertainPerformance I have updated the question with some additional information which differs from the reasoning you shared. Can you help me what is causing the difference? — Nimesh Neema, Mar 02 '20 at 02:39
You say *However, it appears that the aforementioned regular expression matches every line in the file irrespective of its contents.*, but then you say something completely different: *the regular expression matches all the lines containing the pattern ^^ towards the beginning of line.*. Running it myself, I get the same results as your 2nd experience. In what situation were you getting every line matched? — CertainPerformance, Mar 02 '20 at 02:56
@CertainPerformance Thanks for asking. I am using macOS. The former is observed while searching with regex enabled in Sublime Text. The latter was observed when running the grep command using bash shell. I consider myself a regex newbie, so I may be missing some crucial details here. — Nimesh Neema, Mar 02 '20 at 03:01
@CertainPerformance I have also updated the question with some more information. — Nimesh Neema, Mar 02 '20 at 03:04

score 1 · Accepted Answer · edited Mar 16 '20 at 10:32

What ^ matches will depend on the regular expression engine being used. In many languages (including PHP, Python, Javascript, and Java), ^ anywhere outside a character class will always match the start of the string. It will also match the start of a line when the multiline flag is enabled. Notepad++'s regular expressions use Boost, which has the same behavior (except that there's no multiline flag in NP++; ^ will always match the start of a line).

So, here, in NP++, ^^^ means: "Match the position at the start of a line. Then match the position at the start of a line. Then match the position at the start of a line.". Thus, the start of every line gets matched.

In contrast, Bash and a few others which implement BRE flavor of regular expressions treat ^ as an anchor only in certain circumstances:

A circumflex ( '^' ) shall be an anchor when used as the first character of an entire BRE. The implementation may treat the circumflex as an anchor when used as the first character of a subexpression. The circumflex shall anchor the expression (or optionally subexpression) to the beginning of a string; only sequences starting at the first character of a string shall be matched by the BRE. For example, the BRE "^ab" matches "ab" in the string "abcdef", but fails to match in the string "cdefab". The BRE "(^ab)" may match the former string. A portable BRE shall escape a leading circumflex in a subexpression to match a literal circumflex.

In this case, the first ^ is interpreted as matching the beginning of the line, and the next two ^s, since they are not the first character of the pattern, are interpreted as matching literal ^s, rather than as start-of-line anchors.

Different regular expression flavors can have very different behavior, even given the same pattern. This is one of those cases.

What does the regular expression ^^^ mean?

1 Answers1