Regular Expressions provide a much more powerful and efficient way of manipulating strings of text than the use of a variety of standard string functions. They have a reputation of being cryptic and difficult to learn, but are actually quite easy to learn and use.
The RexExp Object
The RegExp object has three properties and three methods. The properties are:
Pattern property - holds the regular expression pattern
Global property - True or False (default False). If False, matching stops at first match.
IgnoreCase property - True or False (default True). If True, allows case-insensitive matching
The methods are:
Execute method - executes a match against the specified string. Returns a Matches collection, which contains a Match object for each match. The Match object can also contain a SubMatches collection
Replace method - replaces the part of the string found in a match with another string
Test method - executes an attempted match and returns True or False
To set up a RegExp object:
Dim reSet re = New RegExp
With re
.Pattern = "some_pattern"
.Global = True
.IgnoreCase = True
End With
A Pattern can be any string value. For example, if the pattern is "Hello World", the RegExp object will match that in the target string. If IgnoreCase is True, it will match any case, so "hellO wORld" would be matched. If Global is set to True, it will contine to search the string for all instances of "Hello World". If False, it will stop searching after the first instance is found.
Execute method, returning Matches collection
Dim re, targetString, colMatch, objMatch
Set rs = New RegExp
With re
.Pattern = "a"
.Global = True
.IgnoreCase = True
End With
targetString = "The rain in Spain falls mainly in the plain"
Set colMatch = re.Execute(targetString)
For each objMatch in colMatch
Response.Write objMatch.Value & "
"
Next
The above will produce a list of 5 letter a's.
Test method, returning True or False
Dim re, targetString
Set rs = New RegExp
With re
.Pattern = "a"
.Global = False
.IgnoreCase = False
End With
targetString = "The rain in Spain falls mainly in the plain"
re.Test(targetString)
The above will return True as soon as it hits the first instance of "a"
Metacharacters
Metacharacters are special characters that can be combined with literal characters (which is all that have been used so far) to extend the power of Regular Expressions way beyond the simple examples already seen, and are what set Regular Expressions apart from simple string functions.
Character
Description
\
Marks the next character as either a special character or a literal. For example, "n" matches the character "n". "\n" matches a newline character. The sequence "\\" matches "\" and "\(" matches "(".
^
Matches the beginning of input.
$
Matches the end of input.
*
Matches the preceding character zero or more times. For example, "zo*" matches either "z" or "zoo".
+
Matches the preceding character one or more times. For example, "zo+" matches "zoo" but not "z".
?
Matches the preceding character zero or one time. For example, "a?ve?" matches the "ve" in "never".
.
Matches any single character except a newline character.
(pattern)
Matches pattern and remembers the match. The matched substring can be retrieved from the resulting Matches collection, using Item [0]...[n]. To match parentheses characters ( ), use "\(" or "\)".
xy
Matches either x or y. For example, "zwood" matches "z" or "wood". "(zw)oo" matches "zoo" or "wood".
{n}
n is a nonnegative integer. Matches exactly n times. For example, "o{2}" does not match the "o" in "Bob," but matches the first two o's in "foooood".
{n,}
n is a nonnegative integer. Matches at least n times. For example, "o{2,}" does not match the "o" in "Bob" and matches all the o's in "foooood." "o{1,}" is equivalent to "o+". "o{0,}" is equivalent to "o*".
{n,m}
m and n are nonnegative integers. Matches at least n and at most m times. For example, "o{1,3}" matches the first three o's in "fooooood." "o{0,1}" is equivalent to "o?".
[xyz]
A character set. Matches any one of the enclosed characters. For example, "[abc]" matches the "a" in "plain".
[^xyz]
A negative character set. Matches any character not enclosed. For example, "[^abc]" matches the "p" in "plain".
[a-z]
A range of characters. Matches any character in the specified range. For example, "[a-z]" matches any lowercase alphabetic character in the range "a" through "z".
[^m-z]
A negative range characters. Matches any character not in the specified range. For example, "[m-z]" matches any character not in the range "m" through "z".
\b
Matches a word boundary, that is, the position between a word and a space. For example, "er\b" matches the "er" in "never" but not the "er" in "verb".
\B
Matches a non-word boundary. "ea*r\B" matches the "ear" in "never early".
\d
Matches a digit character. Equivalent to [0-9].
\D
Matches a non-digit character. Equivalent to [^0-9].
\f
Matches a form-feed character.
\n
Matches a newline character.
\r
Matches a carriage return character.
\s
Matches any white space including space, tab, form-feed, etc. Equivalent to "[ \f\n\r\t\v]".
\S
Matches any nonwhite space character. Equivalent to "[^ \f\n\r\t\v]".
\t
Matches a tab character.
\v
Matches a vertical tab character.
\w
Matches any word character including underscore. Equivalent to "[A-Za-z0-9_]".
\W
Matches any non-word character. Equivalent to "[^A-Za-z0-9_]".
\num
Matches num, where num is a positive integer. A reference back to remembered matches. For example, "(.)\1" matches two consecutive identical characters.
\n
Matches n, where n is an octal escape value. Octal escape values must be 1, 2, or 3 digits long. For example, "\11" and "\011" both match a tab character. "\0011" is the equivalent of "\001" & "1". Octal escape values must not exceed 256. If they do, only the first two digits comprise the expression. Allows ASCII codes to be used in regular expressions.
\xn
Matches n, where n is a hexadecimal escape value. Hexadecimal escape values must be exactly two digits long. For example, "\x41" matches "A". "\x041" is equivalent to "\x04" & "1". Allows ASCII codes to be used in regular expressions.
Examples
\d+ will match any digit one or more times, and is the equivalent to [0-9]+<[^>]*> will match any html tag, and looks for an opening "<", followed by anything that isn't a closing block ">", followed finally by a closing block ">". It uses a "negative character set" [^>]
Constructing a RegExp pattern
Form input validation is a key area in which regular expressions can be used, and a common task is to validate the structure of an email address. Initially, the task needs to be broken down into its constituent rules:
Must have 1 or more letters or numbers
Can have underscores, hyphens, dots, apostrophes
Must have an "@" sign following this
First part of domain name must follow the "@", and must contain at least 3 letters or numbers
May contain underscore, dots or hyphen
Must be at least one dot, which must be followed by the TLD.
"[\w\-\'\.]+@{1}[\w\.?\-?]{3,}\.[\a-z]+" will do it, but can be improved upon depending on how specific you want to be.
SubMatches collection
There will be instances where, once a match is found, you want to extract parts of that match for later use. As an example, suppose you have an html page which contains a list of links: Company A
Company B
Company C
Company D
Company E
...
The required parts are the Company name and the id in the querystring. These need to be collected and inserted into a database, for example. The html is fed in as the strSearchOn, and the pattern uses parenthesis to search for each item - The id ([0-9]{5}), which is a 5 digit number, and ([\w\s]+) which collects a series of letters and spaces, and will stop collecting them when the opening angle bracket is reached (). Set objRegExpr = New regexpobjRegExpr.Pattern = "somepage.asp\?id=([0-9]{5})" & chr(34) & ">([\w\s]+)"objRegExpr.Global = True objRegExpr.IgnoreCase = Trueset colmatches = objRegExpr.Execute(strSearchOn)For Each objMatch in colMatchesid = objMatch.SubMatches(0)company = objMatch.SubMatches(1)sql = "Insert Into table (idfield, company) Values (" & id & ",'" & company & "')"conn.execute(sql)Next
Getting Information about Individual Matches
The MatchCollection object returned by the RegExp.Execute method is a collection of Match objects. It has only two read-only properties. The Count property indicates how many matches the collection holds. The Item property takes an index parameter (ranging from zero to Count-1), and returns a Match object. The Item property is the default member, so you can write MatchCollection(7) as a shorthand to MatchCollection.Item(7).
The easiest way to process all matches in the collection is to use a For Each construct, e.g.:' Pop up a message box for each matchSet myMatches = myRegExp.Execute(subjectString)For Each myMatch in myMatches msgbox myMatch.Value, 0, "Found Match"Next
The Match object has four read-only properties. The FirstIndex property indicates the number of characters in the string to the left of the match. If the match was found at the very start of the string, FirstIndex will be zero. If the match starts at the second character in the string, FirstIndex will be one, etc. Note that this is different from the VBScript Mid function, which extracts the first character of the string if you set the start parameter to one. The Length property of the Match object indicates the number of characters in the match. The Value property returns the text that was matched.
The SubMatches property of the Match object is a collection of strings. It will only hold values if your regular expression has capturing groups. The collection will hold one string for each capturing group. The Count property indicates the number of string in the collection. The Item property takes an index parameter, and returns the text matched by the capturing group. The Item property is the default member, so you can write SubMatches(7) as a shorthand to SubMatches.Item(7). Unfortunately, VBScript does not offer a way to retrieve the match position and length of capturing groups.
Also unfortunately is that the SubMatches property does not hold the complete regex match as SubMatches(0). Instead, SubMatches(0) holds the text matched by the first capturing group, while SubMatches(SubMatches.Count-1) holds the text matched by the last capturing group. This is different from most other programming languages. E.g. in VB.NET, Match.Groups(0) returns the whole regex match, and Match.Groups(1) returns the first capturing group's match. Note that this is also different from the backreferences you can use in the replacement text passed to the RegExp.Replace method. In the replacement text, $1 inserts the text matched by the first capturing group, just like most other regex flavors do. $0 is not substituted with anything but inserted literally
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