preg_match

[edpatterson]

I am rather confused about just how the preg_match functions.
<?
$string1 = '#123&abc';
$string2 = '123abc';
$string3 = '123 456';

preg_match('/[[:alnum:]]/',$string1)
returns 1
preg_match('/[[:alnum:]]/',$string2)
returns 1
preg_match('/[[:alnum:]]/',$string3)
returns 1

I had expected only $string2 to be true.

What am I missing? Just trying to do some basic error checking.

Ed

 

[sleipnir214]

Your pattern matches any single alphanumeric character. preg_match() is indicating, correctly, that each of the three strings has at least one alphanumeric character in it.

I don't understand why you are expecting only $string2 to match. What are you trying to find in the strings?

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[KarveR]

http://uk.php.net/manual/en/function.preg-match.php

this makes good reading, but not at bed-time.

______________________________________________________________________
There's no present like the time, they say. - Henry's Cat.

 

[edpatterson]

I expected a match if the string contained _only_ alphanumeric characters. That is why I thought only 2 would be true.

I read the online manual and that is what lead me to beleive it would work. I also looked it in my my O'Reilly and APress books. I guess I will run to Borders and get a couple more.

Ed

 

[sleipnir214]

The pattern you will want is one that, in English, would read, "A string that is nothing but alphanumeric characters from front to back".

For that, you might try:

preg_match('/^[[:alnum:]]+$/',$string1)

Which would require that the string consist of at least one alphanumeric character. The statement:

preg_match('/^[[:alnum:]]*$/',$string1)

would allow for zero-length strings.

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[edpatterson]

Thanks!
I had tried the + but achived the exact same results, everything was true. Now I am scratching my head as to why the begin of string and end of string markers make a difference.

I'll read and experiment some more to see if I can figure it out.

Ed

 

[KarveR]

because the other strings (1 and 3) have non alphanumeric characters in.


______________________________________________________________________
There's no present like the time, they say. - Henry's Cat.

 

[sleipnir214]

The beginning-of-string and end-of-string characters have to be there. They force preg_match to match the entire string, not just look for a single character withing the string.

Again, your pattern, in English, says, "match a single alphanumeric character". Mine, in English, says, "Start at the beginning of the string. Match alphanumeric characters until you get to the end of the string". If any non-alphanumeric characters appear in the string, preg_match() can't match what I've told it to.

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[edpatterson]

KarveR, you missed the point of my quandry. I realize the other strings have non alphanumeric characters in them, I typed them. I don't understand why the '/[[:alnum]]+/' did not work but the '/^[[:alnum:]]+$/' did.

I think more reading is in order.

Ed

 

[sleipnir214]

edpatterson:

All the pattern '/[[:alnum:]]+/' means is "regardless of the position in the string, match one or more alphanumeric characters in a row". Any non-zero-length string that contains a single alphanumeric character anywhere within it will match the pattern.

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[edpatterson]

DING! the light bulb goes on!

Thanks sleipnir214 for the excellent explaination.

Ed

 

[KarveR]

Sorry Ed, I didn't miss it, I just couldn't think of the best way to explain to you why it wasn't working, when I looked at it I knew why ... Sleipnir strikes again

______________________________________________________________________
There's no present like the time, they say. - Henry's Cat.

 

[sleipnir214]

It just occurred to me that there may be a way of detecting the strings that requires a simpler pattern.

"A string comprised entirely of alphanumeric characters" is the same as "as string that doesn't contain any non-alphanumeric chacters"

This that in mind, perhaps using a pattern of:

/[^[:alnum:]]/

And then checking that the pattern was NOT matched.....

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TANSTAAFL!!