Regex. How can this match?

**Nicholas Paldino [.NET/C# MVP]** · Jul 5 '07, 04:35 PM

Re: Regex. How can this match?

Ethan,

For this, I don't know that I would use that logic. I would parse apart
the parts of the filename by non-alphanumeric characters and then look for
the word or letters in the remaining results. Basically, you would use the
regular expression pattern "\W" (for non-alphanumeric characters) and then
call the Split method on the regular expression, passing your string.

In the array of strings that is returned, look for Forward, Reverse, R
or F.

--
- Nicholas Paldino [.NET/C# MVP]
- mvp@spam.guard. caspershouse.co m

"Ethan Strauss" <ethan dot strauss at Promega dot comwrote in message
news:%23WVSa%23 xvHHA.784@TK2MS FTNGP05.phx.gbl ...

Hi,
>
I have written a regular expression which is supposed to pull a
direction (forward or reverse) designation from a file name.
>
Unfortunately, the direction designation can either be the whole word
("Forward" or "Reverse") or just a single letter ("F" or "R") and the rest
of the name is not as consistent as I would like.. For example
"P1|1_G10_Forwa rd_primer.ab1" or "K8_I1_A01_F.ab 1".
>
At the time I am processing the file names, I have already stripped off
the extension.
>
I have written the Regular Expression
public static Regex DirectionFromFI leName = new
Regex("_(?<Dire ction>[Forward_|Revers e_|R$|F$])");
>
This looks for the underscore, followed by "Forward" or "Reverse" or an
"F" as the last character in the string or an "R" as the last character in
the string, or so I thought.
>
In fact,
>
when Designation = "P1|1_G10_Forwa rd_primer"
>
RegexLibrary.Di rectionFromFIle Name.Match(Desi gnation).Groups["Direction"].Value
= "F"!
>
>
>
How can it pick up that F when it is not the last character? I assume it
has something to do with putting the $ inside the square brackets, but I
can't figure out exactly what it is.
>
>
>
I can figure out a bunch of different work arounds for this, but I would
like to understand what the regular expression is doing for the future.
>
Thanks!
Ethan
>
>
>
>

**Jesse Houwing** · Jul 5 '07, 07:55 PM

Re: Regex. How can this match?

* Ethan Strauss wrote, On 5-7-2007 18:15:

Hi,
>
I have written a regular expression which is supposed to pull a
direction (forward or reverse) designation from a file name.
>
Unfortunately, the direction designation can either be the whole word
("Forward" or "Reverse") or just a single letter ("F" or "R") and the rest
of the name is not as consistent as I would like.. For example
"P1|1_G10_Forwa rd_primer.ab1" or "K8_I1_A01_F.ab 1".
>
I have written the Regular Expression
public static Regex DirectionFromFI leName = new
Regex("_(?<Dire ction>[Forward_|Revers e_|R$|F$])");
>
This looks for the underscore, followed by "Forward" or "Reverse" or an "F"
as the last character in the string or an "R" as the last character in the
string, or so I thought.
>
How can it pick up that F when it is not the last character? I assume it has
something to do with putting the $ inside the square brackets, but I can't
figure out exactly what it is.
>
I can figure out a bunch of different work arounds for this, but I would
like to understand what the regular expression is doing for the future.

There's a few error's in your regex. Let me try to explain:

_ : Find a '_'
[Forward_|Revers e_|R$|F$] : followed by any letter in the
following group 'F','o','r','w'
... ... 's', 'e', '_', '|', 'R', '$'
'F'
(?<Direction>) : Capture these in a named group called
Direction

Of course this isn't what you wanted ;)

This should work better:

_(?<Direction>F orward_|Reverse _|R$|F$)

Just removing the [] would make things work. Now it reads:

_ : Find a '_'
Forward_|Revers e_|R$|F$ : Find either "Forward_", "Reverse_",
"R" followed by end of line,
"F" followed by end of line
(?<Direction>) : Capture these in a named group called
Direction

I present a course in Regular expressions and you've fallen into the
trap many of my students have before you. Be very sure what each kind of
brace, bracket etc means in which context.

() Group, Capture, Set options
[] Character set, any characters in there match
{} Quantifier
<Naming of groups, look around

Jesse
Jesse

**Ethan Strauss** · Jul 6 '07, 02:25 PM

Re: Regex. How can this match?

Thanks Jesse,
I had actually figured it out and was about to post the answer, but you
beat me to it.
The Regex which ended up working as I wanted is
"_(?<Direction> Forward|Reverse |R$|F$)"

This is slightly different from what is below, but only because I
changed my mind about what characters to capture...
Ethan

"Jesse Houwing" <jesse.houwing@ nospam-sogeti.nlwrote in message
news:uVrv90zvHH A.3560@TK2MSFTN GP02.phx.gbl...

>* Ethan Strauss wrote, On 5-7-2007 18:15:

>Hi,
>>
> I have written a regular expression which is supposed to pull a
>direction (forward or reverse) designation from a file name.
>>
> Unfortunately, the direction designation can either be the whole word
>("Forward" or "Reverse") or just a single letter ("F" or "R") and the
>rest of the name is not as consistent as I would like.. For example
>"P1|1_G10_Forw ard_primer.ab1" or "K8_I1_A01_F.ab 1".
>>
> I have written the Regular Expression
>public static Regex DirectionFromFI leName = new
>Regex("_(?<Dir ection>[Forward_|Revers e_|R$|F$])");
>>
>This looks for the underscore, followed by "Forward" or "Reverse" or an
>"F" as the last character in the string or an "R" as the last character
>in the string, or so I thought.
>>
>How can it pick up that F when it is not the last character? I assume it
>has something to do with putting the $ inside the square brackets, but I
>can't figure out exactly what it is.
>>
>I can figure out a bunch of different work arounds for this, but I would
>like to understand what the regular expression is doing for the future.

>
There's a few error's in your regex. Let me try to explain:
>
_ : Find a '_'
[Forward_|Revers e_|R$|F$] : followed by any letter in the
following group 'F','o','r','w'
... ... 's', 'e', '_', '|', 'R', '$'
'F'
(?<Direction>) : Capture these in a named group called
Direction
>
Of course this isn't what you wanted ;)
>
This should work better:
>
_(?<Direction>F orward_|Reverse _|R$|F$)
>
Just removing the [] would make things work. Now it reads:
>
_ : Find a '_'
Forward_|Revers e_|R$|F$ : Find either "Forward_", "Reverse_",
"R" followed by end of line,
"F" followed by end of line
(?<Direction>) : Capture these in a named group called
Direction
>
I present a course in Regular expressions and you've fallen into the trap
many of my students have before you. Be very sure what each kind of brace,
bracket etc means in which context.
>
() Group, Capture, Set options
[] Character set, any characters in there match
{} Quantifier
<Naming of groups, look around
>
Jesse
Jesse

Regex. How can this match?

Regex. How can this match?

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