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:
Set 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 re = 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 & "<br />" Next
The above will produce a list of 5 letter a's.
Test method, returning True or False
Dim re, targetString Set re = 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 "\(" or "\)". |
| x|y | Matches either x or y. For example, "z|wood" matches "z" or "wood". "(z|w)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:
<a href="somepage.asp?id=12345">Company A</a><br /> <a href="somepage.asp?id=45678">Company B</a><br /> <a href="somepage.asp?id=66745">Company C</a><br /> <a href="somepage.asp?id=33471">Company D</a><br /> <a href="somepage.asp?id=90765">Company E</a><br /> ...
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 (</a>).
Set objRegExpr = New regexp
objRegExpr.Pattern = "somepage.asp\?id=([0-9]{5})" & chr(34) & ">([\w\s]+)"
objRegExpr.Global = True
objRegExpr.IgnoreCase = True
set colmatches = objRegExpr.Execute(strSearchOn)
For Each objMatch in colMatches
id = objMatch.SubMatches(0)
company = objMatch.SubMatches(1)
sql = "Insert Into table (idfield, company) Values (" & id & ",'" & company & "')"
conn.execute(sql)
Next
