Slashes and regular expressions

delphi

Hi,

I hope you can help me understand this. I know how regular expressions work, although I don't use them very often. I know that the slash '\' is used as an escape character, so if you wanted to match a period, it would be . and a escaped backslash would be \.

Okay, here's where my confusion starts. Sometimes in the "eregi" functions I see a DOUBLE slash as an escape. In other words, instead of . I see \. to escape a period (or any other special character).

Can someone explain to me why the double slash is needed? I'm thinking it has something to do with the "magic quotes" setting, but if not, then I'm really confused as to why.

Thanks

rheroux

If you see something like \. in a regular expression, it means it's looking for a \ followed by any non-newline character.

So, \. could match \f \b \1 \5, etc.

SOS

Or the string is in double-quotes(that enables normal escape characters) so the double-backslash \ becomes a single backslash.

delphi

Hi guys,

Thanks for the reply, but it doesn't seem to answer my question. I'll include a code sample this time, so maybe I can be more clear.

This is from the popular phpBook Guest book script that is available. I am actually doing my own guest book, but I came across this one, and found the regex validation and was looking at it.

Here's where I am cofused:

$msg = eregi_replace("\[url\]www.([^{\[]*)\[/url\]",} "replace ment stuff");

looking at the left side (I didn't copy the replacement regex), I see that there is "\[", but my question is why shouldn't this be "[". It is for replacing the popular VBB propreitary URL tags.

I have a Windows-based POSIX/PHP regex testing program, and that code doesn't match, however, the following does, and it makes more sense to me:

[url]www.([^[]*)[/url]

Notice, all I did was remove the double slashes and replace them with a single slash. So back to my original question, how come there are double slashes to escape the '[' and ']' brackets? Doesn't make sense to me. I've seen this in other people's code too in the past. The only viable solution that makes sense relates to the "magic_quotes" setting.

Thanks

SOS

What I said. The \ is there to prevent PHP from thinking it is an escape character.
I suppose that's related to "magic quotes", whatever they may be.