RegExp练习：带有先行断言的不情愿量词

Question

Can you explain me how this works? Here is an example:

<!-- The quick brown fox 
              jumps over the lazy dog -->

<!--[if IE 7]>
    <link rel="stylesheet" type="text/css" href="/supersheet.css" />
<![endif]-->

<!-- Pack my box with five dozen liquor jugs -->

First, I tried to use the following regular expression to match the content inside conditional comments:

/<!--.*?stylesheet.*?-->/s

It failed, as the regular expression matches all the content before the first <!-- and the last --> . Then I tried using another pattern with a lookahead assertion:

/<!--(?=.*?stylesheet).*?-->/s

It works and matches exactly what I need. However, the following regular expression works as well:

/<!--(?=.*stylesheet).*?-->/s

The last regular expression does not have a reluctant quantifier in the lookahead assertion. And now I am confused. Can anyone explain me how it works? Maybe there is a better solution for this example?

Updated:

I tried usig the regular expressions with lookahead assertion in another document, and it failed to mach the content between the comments. So, this one /<!--(?=.*?stylesheet).*?-->/s (as well as this one /<!--(?=.*stylesheet).*?-->/s ) is not correct. Do not use it and try other suggestions.

Updated:

The solution has been found by Jonny 5 (see the answer). He suggested three options:

1. Using of a negated hyphen to limit match. This option works only if there is no a hyphen between the tags. If a stylesheet has an URL /style-sheet.css , it will not work.
2. Using of escape sequence: \\K . It works like a charm. The downsides are the following:
   • It is terribly slow (in my case, it was 8-10 times slower than the other solutions)
   • Only available since PHP 5.2.4
3. Using a lookahead to narrow the match. This is the goal I tried to achieve, but my expirience of using lookaround assertions was insufficient to perform the task.

I think the following is a good solution for my example:

/(?s)<!--(?:(?!<!).)+?stylesheet.+?-->/

The same but with the s modifier at the end:

/<!--(?:(?!<!).)+?stylesheet.+?-->/s

As I said, this is a good solution, but I managed to improve the pattern and found another one that in my case works faster.

So, the final solution is the following:

/<!--(?:(?!-->).)+?stylesheet.+?-->/s

Thanks all the participants for interesting answers.

Answer 1

The string stylesheet is mentioned only one time in your test document, so both regular expressions you tried will match the same thing but in different ways.

<!--(?=.*?stylesheet).*?-->/s

This one does the following:

• Capture <!-- .
• Look ahead, capturing characters up to and including stylesheet . Fail if not found.
• Capture characters up to and including --> .

<!--(?=.*stylesheet).*?-->/s

This one does the following:

• Capture <!-- .
• Look ahead, capturing any character until no longer possible. Backtrack, continuously trying to match stylesheet . Fail if not found.
• Capture characters up to and including --> .

Basically, one needs to backtrack significantly while the other doesn't.

If your subject instead is...

<!-- The quick brown fox 
              jumps over the lazy dog -->

<!--[if IE 7]>
    <link rel="stylesheet" type="text/css" href="/supersheet.css" /> <![endif]-->

<!-- Pack my box with five dozen stylesheets -->

you get two different results. The former would find the first stylesheet , while the latter would find the second (and last) since it starts searching from the end of the string.

Answer 2

To match only the part <!-- ... stylesheet ... --> there are many ways:

1.) Use a negated hyphen [^-] to limit the match and stay in between <!-- and stylesheet

(?s)<!--[^-]+stylesheet.+?-->

[^-] allows only characters, that are not a hyphen. See test at regex101 .

---

2.) To get the "last" or closest match without much regex effort, also can put a greedy dot before to ᗧ eat up. Makes sense if not matching globally / only one item to match. Use \\K to reset after the greed:

(?s)^.*\K<!--.+?stylesheet.+?-->

See test at regex101 . Also can use a capture group and grab $1: (?s)^.*(<!--.+?stylesheet.+?-->)

---

3.) Using a lookahead to narrow it down is usually more costly:

(?s)<!--(?:(?!<!).)+?stylesheet.+?-->

See test at regex101 . (?!<!). looks ahead at each character in between <!-- and stylesheet if not starting another <! ... to stay inside one element. Similar to the negated hyphen solution.

---

Instead of .* I used .+ for one or more - depends on what to be matched. Here + fits better.
What solution to use depends on the exact requirements. For this case I would use the first.