Regular Expressions in VBScript - Constructing Patterns
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At this point regular expressions don’t seem any better that VBS’s own string functions. In fact, they only seem to take more code! But that’s because you haven’t seen the magic of patterns yet. What if you wanted to know how many words were in a sentence?
strTest = "This is my test string."
Set objRegExp = New RegExp
objRegExp.Global = True
objRegExp.IgnoreCase = True
objRegExp.Pattern = "w+"
Set colMatches = objRegExp.Execute(strTest)
WScript.Echo colMatches.Count
Or which words begin with the letter “t”?
strTest = "This is my test string."
Set objRegExp = New RegExp
objRegExp.Global = True
objRegExp.IgnoreCase = True
objRegExp.Pattern = "bt[a-z]+b"
Set colMatches = objRegExp.Execute(strTest)
For Each objMatch In colMatches
WScript.Echo objMatch.Value
Next
You can quickly see how patterns can make all of the difference. But what are patterns and how do you make them? A pattern is a string of literal characters to be matched. However, there are a series of reserved and escaped characters that can be used to control match positions, occurrences, wild cards, and more. We’ll begin with match positions as listed in Table 1 below.
Table 1: Position Matching
Symbol | Description |
^ | Matches the beginning of a string. “^This” would match the word “This” if it appeared at the beginning of a string. |
$ | Matches the end of a string. “.$” matches the period at the end of a string. |
b | Matches a word boundary. “bt” matches the letter t at the beginning of a word. |
B | Matches a non-word boundary. “BxB” matches any letter x that does not appear at the beginning or end of a word. |
After positioning, you’ll want to match literal characters. Alphanumeric characters are treated as literals. However, some of them have special meanings. Those characters must be escaped by a back-slash.
Table 2: Matching Literals
Symbol | Description |
Alphanumeric | Matches any alphanumeric character literally. |
n | Matches a new line |
f | Matches a form feed |
r | Matches a carriage return |
t | Matches horizontal tab |
v | Matches a vertical tab |
? | Matches a ? |
* | Matches a * |
+ | Matches a + |
. | Matches a . |
| | Matches a | |
{ | Matches a { |
} | Matches a } |
| Matches a |
[ | Matches a [ |
] | Matches a ] |
( | Matches a ( |
) | Matches a ) |
xxx | Matches the ASCII character expressed by the Octal number. “50” matches “(“ or Chr(40) |
xdd | Matches the ASCII character expressed by the Hex number. “x28” matches “(“ or Chr(40) |
uxxxx | Matches the ASCII character expressed by the Unicode number. “u00A3” matches “£” |
Once you have the ability to match literal characters, you’ll probably find the need to expand a bit. You may want to match any one character in a range of characters, or perhaps everything except a specified character. This is done with character classes.
Table 3: Matching Character Classes
Symbol | Description |
[xyz] | Matches any character is the character set. Hyphens denote ranges. “[a-z]” matches any character a through z |
[^xyz] | Matches any character not in the character set. “[^0-9] matches any non-digit character |
. | Matches any character except n. |
w | Match any word character. Equivalent to [a-zA-Z_0-9] |
W | Match any non-word character. Equivalent to [^a-zA-Z_0-9] |
d | Match any digit. Equivalent to [0-9] |
D | Match any non-digit character. Equivalent to [^0-9] |
s | Match any whitespace character. Equivalent to [ trnvf] |
S | Match any non-whitespace character. Equivalent to [^ trnvf] |
At this point, your patterns will still be matching one character at a time. To unleash the power of regular expressions, you need to match repeating characters.
Table 4: Matching Repetition
Symbol | Description |
{x} | Matches x occurrences. “d{5}” matches 5 digits. |
{x,} | Matches x or more occurrences. “d{2,}” matches 2 or more consecutive digits. |
{x,y} | Matches x to y occurrences. “d{2,3}” matches no less than two digits and no more than three. |
? | Matches 0 or 1 occurrence. Equivalent to {0, 1}. “d?” matches 0 or 1 digit. |
* | Matches 0 or more occurrences. Equivalent to {0,}. “d*” matches 0 or more digits. |
+ | Matches 1 or more occurrences. Equivalent to {1,}. “d+” matches 1 or more digits. |
Finally, grouping and alternation offer the ability to make extremely complex regular expressions. Grouping allows you to match clauses. Alternation allows you to add more than one clause and match any one of them.
Table 5: Grouping and Alternation
Symbol | Description |
() | Grouping creates a clause. Clauses may be nested. “(ab)?(c)” matches “abc” or “c”. |
()|() | Alternation groups clauses into one expression and then matches any one of the clauses. “(ab)|(cd)|(ef)” matches “ab”, “cd”, or “ef”. |
Regular expressions also allow a feature called back referencing. Back referencing allows you to reuse part of an expression. This is done by providing a back-slash followed by a digit. For example, the expression “(w+)s+1” matches any one word that occurs twice in a row. In other words, the same match must be made twice in a row.
Next: Building useful patterns >>
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