Re: Re: Common Regex Gotchas

in reply to Re: Common Regex Gotchas
in thread Common Regex Gotchas

Sorry - re-reading it, I realized that I accidently inserted html tags that are not showing up.

I found the Greedy section to be quite confusing. First of all, I think you probably have a couple slash-s'es in your html that is causing it to not print and makes the last couple paragraphs very difficult to understand and read.

Also, I'm still confused about why there is a match at all in the first example. Why doesn't the engine continue backwards past the whitespace and look for a <\/tag> string?

Finally, why does the last example (still in the Greedy section) work? If, when creating the example string, I carraige return after the <\/tag>, there shouldn't be a whitespace to match on, right?

Finally, finally, thanks for putting this together... it's really speeding my ramp along...

Comment on Re: Re: Common Regex Gotchas

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Re: Re: Re: Common Regex Gotchas by chromatic (Archbishop) on Nov 27, 2001 at 04:47 UTC
Why doesn't the engine continue backwards past the whitespace and look for a <\/tag> string? Because the engine prefers the longest match that starts at the leftmost possible position. When it hits ., it jumps all the way to the end of the string and then backtracks, trying to match the next necessary character. Because it's backtracking, it matches </tag> at the end of the string. That fits the pattern, so it doesn't continue backtracking to find a shorter match. If, when creating the example string, I carraige return after the <\/tag>, there shouldn't be a whitespace to match on, right?* The /s flag allows the '.' token to match newlines. Adding the minimal token '?' avoids the jump-to-end-then-backtrack behavior. It works like you'd expect, trying to match as few characters as possible. Does that clear it up? I've also touched up the formatting somewhat.	[reply]

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Re: Re: Re: Common Regex Gotchas
by chromatic (Archbishop) on Nov 27, 2001 at 04:47 UTC

Why doesn't the engine continue backwards past the whitespace and look for a <\/tag> string?

Because the engine prefers the longest match that starts at the leftmost possible position. When it hits .*, it jumps all the way to the end of the string and then backtracks, trying to match the next necessary character. Because it's backtracking, it matches </tag> at the end of the string. That fits the pattern, so it doesn't continue backtracking to find a shorter match.

If, when creating the example string, I carraige return after the <\/tag>, there shouldn't be a whitespace to match on, right?

The /s flag allows the '.' token to match newlines. Adding the minimal token '?' avoids the jump-to-end-then-backtrack behavior. It works like you'd expect, trying to match as few characters as possible.

Does that clear it up? I've also touched up the formatting somewhat.

[reply]

In Section Tutorials