Regular Expression.

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# 1  
Old 06-19-2011
Regular Expression.

can someone let me know what this means in english.


\ is an escape key right?


Also im getting confused with something like it

[a-z] does this mean any single character?
and this would be 2 characters [a-z][a-z]?

Just let me know if im on the right track.
# 2  
Old 06-19-2011
Hi Syco

\ is a escape character. This is used to escape the special meaning of the character.
As * is a meta character, If you do not want to use it as a meta character 
use like this

$ echo \*

[ ] is used for the range.
[a-z] means --> a character within a to z included a and z
i.e - c

[a-z][a-z] --> two times , like
i.e- pr

hope its ok.
This User Gave Thanks to atul9806 For This Post:
# 3  
Old 06-19-2011
[a-z] means any combination of letters in lower case and [A-Z] means any combination of letters in upper case.

---------- Post updated at 02:57 PM ---------- Previous update was at 02:49 PM ----------

Sorry Smilie I am wrong.
Thanks atul.
# 4  
Old 06-19-2011

so with the above does it just simply mean abcd?

---------- Post updated at 05:53 AM ---------- Previous update was at 05:53 AM ----------


so with the above does it just simply mean abcd?
# 5  
Old 06-19-2011
In a script, I want to print (abcd) -- with the parentheses
echo (abcd)

$ ./test.s
./test.s: line 1: syntax error near unexpected token `abcd'
./test.s: line 1: `echo (abcd)'
echo \(abcd\)     

$ ./test2.s

# 6  
Old 06-19-2011
In a regular expression parentheses are used to group characters, so you need to escape them if you mean to use them as characters to match.

\(abcd\) will match occurrences of "(abcd)".
(abcd) will match occurrences of "abcd".

You would group characters if you want to capture them, for example so you use them in a substitution, or to simplify expressions.

For example:

a(bc)?d will match "ad" or "abcd".
r(eig|ai)n will match "reign" or "rain".

Without being able to group characters in those expression you would otherwise have to offer the full version of every pattern. This is not a problem with those where there are only two possible variants, but with more complicated expressions you could otherwise end up with very long expressions.

If you see a character escaped though it essentially means to use it as a literal character rather than a special one.

So while [a-z] means match any character in the range from "a" to "z", you could also have \[a-z\] to match actual occurrences of "[a-z]".

This User Gave Thanks to mij For This Post:
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