regular expression question

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Ludwig

Hi,

i'm using the regular expression \b\w to find the beginning of a word,
in my C# application. If the word is 'public', for example, it works.
However, if the word is '<public', it does not work: it seems that <
is not a valid character, so the beginning of the word starts at
theletter 'p' instead of '<'.

Because I'm not an expert in regular expressions, maybe someone of you
guys can help me? I need the correct regex to find the beginning of
the word '<public' in a string.

Thanks...

Kind regards,
Ludwig
 
Hi Ludwig,

It is not possible to answer your question as you've stated it. Here's why:
i'm using the regular expression \b\w to find the beginning of a word,
in my C# application. If the word is 'public', for example, it works.
However, if the word is '<public', it does not work: it seems that <
is not a valid character, so the beginning of the word starts at
theletter 'p' instead of '<'.
Click to expand...

You have not defined your terms. You use the word "word," but you have not
defined what that is supposed to mean in your situation. In regular
expressions, there are no words, only characters. The "\w" character class
indicates a word *character*. A word character is defined in regular
expressions as a character that is either a digit or a letter of the
alphabet.

So, the character '<' is not defined in regular expressions as a word
character, and therefore is not identified as belonging to the set defined
by your rule.

However, while you have stated that you *do* want to identify the character
'<' as the "beginning of a word," you have not stated exactly what the rule
is, only a small part of it. For example, by what you've told me, the
following character sequences could all be "words" -

Hello Ludwig ('H', 'L') The first letters of each word are identified.

Hello, <Ludwig> ('H', '<') The first letter of "Hello" and the beginning '<'
are identified.

Hello, !!!!!!! ('H', '!') The first letter of "Hello" and the beginning '!'
are identified. This is possible because you have not stated what characters
you do *not* consider to be the beginnings of words.

And so on. In other words, a regular expression is shorthand for a rule that
defines a pattern. You need to explicitly define what the rule is in order
for me to create a regular expression that satisfies that rule.

--
HTH,

Kevin Spencer
Microsoft MVP
Professional Numbskull

Show me your certification without works,
and I'll show my certification
*by* my works.
 
Hi Ludwig,

It is not possible to answer your question as you've stated it. Here's why:


You have not defined your terms. You use the word "word," but you have not
defined what that is supposed to mean in your situation. In regular
expressions, there are no words, only characters. The "\w" character class
indicates a word *character*. A word character is defined in regular
expressions as a character that is either a digit or a letter of the
alphabet.

So, the character '<' is not defined in regular expressions as a word
character, and therefore is not identified as belonging to the set defined
by your rule.

However, while you have stated that you *do* want to identify the character
'<' as the "beginning of a word," you have not stated exactly what the rule
is, only a small part of it. For example, by what you've told me, the
following character sequences could all be "words" -

Hello Ludwig ('H', 'L') The first letters of each word are identified.

Hello, <Ludwig> ('H', '<') The first letter of "Hello" and the beginning '<'
are identified.

Hello, !!!!!!! ('H', '!') The first letter of "Hello" and the beginning '!'
are identified. This is possible because you have not stated what characters
you do *not* consider to be the beginnings of words.

And so on. In other words, a regular expression is shorthand for a rule that
defines a pattern. You need to explicitly define what the rule is in order
for me to create a regular expression that satisfies that rule.
Click to expand...

Thanks for the explaination, Kevin!

Well, I'm working on a editor control that supports syntax
highlighting. I have al list of words that should be highlighted when
typed in the editor, for example 'public', 'class', etc.

So at a given time, the user types in the word public, and when the
character 'c' is typed, the word 'public' is colored in blue, for
example.

At the moment, I use the pattern '\b\w' to identify the first
character of the 'word' in the editor, and I use '\w\b' to identify
the last character of a word. This works.

However, there are also xml tags that need to be highlighted; for
example, <sometagname> : if the user types in the '<', it should be
colored; if he then types the last 'e' of 'sometagname', the word
'sometagname' should be colored, if he then types '>', that too should
be colored.

So in fact, each word or character that I define in the list of words,
should be colored.

This list of words can be (for example): public, class, int, long,
byte, byte[], <, >, sometagname, generic<>, etc....

So, if I try to define the rule:
- spaces always define the beginning and end of a word:
public class Test() -> I need to identify the public, class, Test()
- there are characters that are not seperated by spaces but that also
have to be found when typed:
<?xml version="1.0" encoding="utf-8" ?> -> I need to identify the <,
?, xml, version, encoding in order to highlight these in various
colors.

I hope that you understand what I'm trying to do here...

Kind regards,
Ludwig
 
Hi Ludwig,

I can understand that you're trying to implement syntax highlighting in
your application.

What I don't understand is how you can use \b\w to catch a particular
word in your list. \b matches a word boundary, so that's ok, but \w
will match any word like character.

So to catch 'public' , if you used "\b\wpublic", it would not match,
since the 'p' of public has already been matched by '\w'. Even tried it
with positive Lookbehind, but still doesn't work.

Unless the words in your list are something like : "ublic", "lass",
"yte"

Kindly clarify,

Regards,

Cerebrus.
 
Hi Ludwig,

We're getting closer, but remember that close only counts in horseshoes and
hand-grenades, not in programming!

To use Regular Expressions, you must be *absolutely specific* about your
rules.
Well, I'm working on a editor control that supports syntax
highlighting. I have al list of words that should be highlighted when
typed in the editor, for example 'public', 'class', etc.

So at a given time, the user types in the word public, and when the
character 'c' is typed, the word 'public' is colored in blue, for
example.
Click to expand...

Let me explain what is missing here. "Syntax" means nothing to Regular
Expressions, and very little to humans. That is, it can refer to so many
different things (such as the "syntax" I'm using to write this post) that it
identifies nothing in and of itself.
I have al list of words that should be highlighted when
typed in the editor
Click to expand...

That is what you think you mean, but that is not what you mean. For example,
note the 2 uses of "public" in the following example:

public string Opened()
{
return "Open to the public";
}

Now, the first instance of "public" is syntax, but the second is part of a
string. In other words, "syntax" is a set of rules. From Dictionary.com,
"syntax" means:

"The rules governing the formation of statements in a programming language."

Obviously, "public" as part of a string is not syntax. How do you expect to
tell the Regular expression the difference? You must know the exact syntax
rules, and be able to express them in Regular Expression syntax.
At the moment, I use the pattern '\b\w' to identify the first
Click to expand...

Of course, this is unsuitable. The "\b" expression indicates the beginning
or ending or a word, that is a set of characters that is composed entirely
of word characters, and as I said before, '<' is not a word character.
However, there are also xml tags that need to be highlighted; for
example, <sometagname> : if the user types in the '<', it should be
colored; if he then types the last 'e' of 'sometagname', the word
'sometagname' should be colored, if he then types '>', that too should
be colored.
Click to expand...

Okay, now you've introduced the topic of XML, which was not part of the
topic in your earlier message, nor up until this point in your current post.
Yet, you have not stated what you mean by "syntax highlighting," nor what
this "syntax" is for. I could assume that you mean "XML syntax" but you have
not said so, so I cannot logically make that assumption. The string you're
parsing may only *contain* XML, as well as other "syntax."
So in fact, each word or character that I define in the list of words,
should be colored.
Click to expand...

Not necessarily. See my example (about "public") above. You need to be
*absolutely specific*.
- spaces always define the beginning and end of a word:
Click to expand...

Are you certain of this? What about line breaks? Might any of these "words"
be at the beginning or end of the string? If so, they will either not be
preceded by a space nor followed by one.
<?xml version="1.0" encoding="utf-8" ?> -> I need to identify the <,
?, xml, version, encoding in order to highlight these in various
colors.
Click to expand...

Okay, see, now you want to identify the '?' in an XML tag. But that is not a
word character, nor is it delimited from "xml" by a space. Again, the syntax
of the Regular Expression depends upon an *absolutely specific* description
of the rules for matching and grouping.
I hope that you understand what I'm trying to do here...
Click to expand...

Not yet, but I hope to!

--
HTH,

Kevin Spencer
Microsoft MVP
Professional Numbskull

Show me your certification without works,
and I'll show my certification
*by* my works.
 
Hi Ludwig,

We're getting closer, but remember that close only counts in horseshoes and
hand-grenades, not in programming!

To use Regular Expressions, you must be *absolutely specific* about your
rules.


Let me explain what is missing here. "Syntax" means nothing to Regular
Expressions, and very little to humans. That is, it can refer to so many
different things (such as the "syntax" I'm using to write this post) that it
identifies nothing in and of itself.


That is what you think you mean, but that is not what you mean. For example,
note the 2 uses of "public" in the following example:

public string Opened()
{
return "Open to the public";
}

Now, the first instance of "public" is syntax, but the second is part of a
string. In other words, "syntax" is a set of rules. From Dictionary.com,
"syntax" means:

"The rules governing the formation of statements in a programming language."

Obviously, "public" as part of a string is not syntax. How do you expect to
tell the Regular expression the difference? You must know the exact syntax
rules, and be able to express them in Regular Expression syntax.

Of course, this is unsuitable. The "\b" expression indicates the beginning
or ending or a word, that is a set of characters that is composed entirely
of word characters, and as I said before, '<' is not a word character.
Click to expand...
Okay, now you've introduced the topic of XML, which was not part of the
topic in your earlier message, nor up until this point in your current post.
Yet, you have not stated what you mean by "syntax highlighting," nor what
this "syntax" is for. I could assume that you mean "XML syntax" but you have
not said so, so I cannot logically make that assumption. The string you're
parsing may only *contain* XML, as well as other "syntax."

Not necessarily. See my example (about "public") above. You need to be
*absolutely specific*.

Are you certain of this? What about line breaks? Might any of these "words"
be at the beginning or end of the string? If so, they will either not be
preceded by a space nor followed by one.

Okay, see, now you want to identify the '?' in an XML tag. But that is not a
word character, nor is it delimited from "xml" by a space. Again, the syntax
of the Regular Expression depends upon an *absolutely specific* description
of the rules for matching and grouping.


Not yet, but I hope to!
Click to expand...

Hi Kevin,

thanks again! Okay, seems like I need to explain further what I need
:) For my application, I need a .NET textbox control where the user
can type XML, XSTL or HTML. And, it would be nice that the control can
do syntax coloring (and intellisense, later on), just like visual
studio does.

So I did a little test, by inheriting from RichTextBox control,
overriding OnTextChanged() and implementing something that already did
some syntax coloring with seperate words like 'public', 'class' etc;
but obviously, after your replies I now see that I did not define the
rules specific enough. The word 'public' in a string is not a keyword,
indeed. However, this test allowed me find away to completely avoid
flickering of the control, and now the next step is to do the syntax
coloring, with the rules of XML.

What the user enters into the textbox can be simple xml, like:

<?xml version="1.0" encoding="ISO-8859-1"?>
<note>
<to>Kevin</to>
<from>Ludwig</from>
<heading>Thank you</heading>
<body>Thanks for helping me!!</body>
</note>

or XSLT:

<?xml version="1.0" encoding="ISO-8859-1"?><xsl:stylesheet
version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"><xsl:template
match="/">
<html>
<body>
<h2>My CD Collection</h2>
<table border="1">
<tr bgcolor="#9acd32">
<th align="left">Title</th>
<th align="left">Artist</th>
</tr>
<xsl:for-each select="catalog/cd">
<tr>
<td><xsl:value-of select="title"/></td>
<td><xsl:value-of select="artist"/></td>
</tr>
</xsl:for-each>
</table>
</body>
</html>
</xsl:template></xsl:stylesheet>

The idea is, that when the user types a character, the current caret
position in the textbox is taken to analyze the surrounding
words/characters, to see that there's a valid xml tag is formed, like
<body>, or <xsl:for-each select="catalog/cd">. If a valid tag has
formed, the tag name (body, xsl:for-each, select) should be colored in
a specific color (if it's in a list of valid tag names, but maybe we
can skip this for now). Also the < and > have to be colored in another
color. Attrributes also get another color (version, encoding, select).
Literals and other not defined xml elements like 'My CD Collection' do
not need coloring.

If the user deletes a character so that a tag becomes an invalid tag
(for example, deleting the >), then the coloring of the incomplete tag
has to be removed.

I hope that this time I explained the context better... didn't realize
that I had to be that specific.... so i's all about xml rules.
 
Hi Ludwig,

I can understand that you're trying to implement syntax highlighting in
your application.

What I don't understand is how you can use \b\w to catch a particular
word in your list. \b matches a word boundary, so that's ok, but \w
will match any word like character.

So to catch 'public' , if you used "\b\wpublic", it would not match,
since the 'p' of public has already been matched by '\w'. Even tried it
with positive Lookbehind, but still doesn't work.

Unless the words in your list are something like : "ublic", "lass",
"yte"

Kindly clarify,

Regards,

Cerebrus.
Click to expand...

Hi Cerebrus,

well, imagine the user types in:

public class

when the last 's' is typed, I use the regular expression \b\w in a
right-to-left-search to find the beginning of the word class, and then
I use the regular expression \w\b, starting from the position of this
word, to find the end of the word. This way I have found the word
'class' and check if it's a keyword, and color it.

This works for 'words', but of course, I did not define it enough,
because the word class is not always a keyword, for example when used
in a string.

So this would never work for the xml stuff I'm trying to do (see my
last reply to Kevin in this thread).

Thanks!
 
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