using regular expressions in C on linux

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Hi Team,
Can you please tell what is the meaning of this particular pattern.
I am just being a beginner and found this as an example in one of the site
providing example for reg expression through regex.h

char *PatternSearch = "[?&]XYZ=\\([[:alnum:]]*\\)";

Appreciate your response if you could explain what does this pattern look for with an example

Regards,

Posted 30-Oct-11 5:18am

AshakiranBhatter

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Andrew Brock · Answer 1 · 2011-10-30T05:34:00

Solution 1

[?&] is one of ? or &
XYZ= is just that exact text (case sensitive)
\\( is the opening bracket character (. The \\ is escaped by the compiler, so it is passing \( into the regex
[:alnum:] is shorthand for A-Za-z0-9 (no square brackets) which expands to ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789. Note that the [] is part of this expansion.
[[:alnum:]] searches for [A-Za-z0-9]
[[:alnum:]]* searches for any number of [[:alnum:]] (see previous)
\\) is the closing bracket character ). Again \\ is escaped by the compiler, passing \( in as the regex

So, it is searching for

C++

?XYZ=(/*any characters in here*/)

or

C++

&XYZ=(/*any characters in here*/)

Posted 30-Oct-11 5:34am

Andrew Brock

Comments

[no name] 30-Oct-11 11:41am

thank you very much

[no name] 30-Oct-11 11:42am

but what is the difference between [:alnum:] and [[:alnum:]]

Andrew Brock 30-Oct-11 11:52am

[:alnum:] expands to JUST "A-Za-z0-9", not "[A-Za-z0-9]". Since you want it as any character in the set, you need to add the extra [] around it to get it to "[A-Za-z0-9]".
This allows you to do something like "[[:alnum:]_]" which will also include an underscore character in the search.

[no name] 30-Oct-11 12:00pm

please correct me if I am wrong.
So [[:alnum:]] means only [a] or [abcd] aswell. Want to know how many characters i can repeat through this pattern.

Andrew Brock 30-Oct-11 12:05pm

The full expansion of "[[:alnum:]]" is "[ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789]".
This will match any single upper or lower case character or any single digit.
It is then followed by a * in your regex, which means that there can be 0 or more of these. So "[[:alnum:]]*" will match any combination of numbers and letters

[no name] 30-Oct-11 12:01pm

what i mean is only one character or I can include multiple characters in [ ]

[no name] 30-Oct-11 12:10pm

So [[:alnum:]]* will match a string like [a][b][c][d][e][1][2][3]
Is this correct.
Sorry for asking the same thing many times. As I wanted to learn i am doing this.

Andrew Brock 30-Oct-11 12:15pm

"[[:alnum:]]" will match the following strings, to name a few:
A
B
C
a
b
c
0
1
2
and any other single letter or digit.

"[[:alnum:]]*" will match the following strings:
<empty string>
A
b
0
AA
AAA
AAB
AAC
AAa
AAb
aa
aAa
a01
A0a
aA0sad0asdasDA
And anything else you can make out of any number of letters and digits.

[no name] 30-Oct-11 12:24pm

but [[:alnum:]]* = "[a][b]"
where as [:alnum:]* = "ab"
you have just mentioned in previous point.
little bit of confusion here. Please clarify

Andrew Brock 30-Oct-11 12:33pm

I'm not sure where you got that from. I don't see anywhere that I said that, and it is not what I ever intended.
"[a][b]" would have no logical sense. It is exactly the same as "ab". "[a]" is giving it the option of choosing from 1 character.

[no name] 30-Oct-11 13:15pm

in this regards, then [[:alnum:]]* = "[ab]" and [:alnum:]* = "ab"
can i assume that the above strings matches to the pattern ?