In their simplest form a Regular Expression is simply a description of the pattern we are trying to match. This pattern is placed inside a pair of
/'s, for example in the replacement example on the previous page, our Regular Expression could be :
replace(/2002/,"2003")
However, Regular Expressions offer us shortcuts to represent specific groupings of character, for example :
| Regular Expression |
Description |
| \d |
Matches any digit - 0,1,2,3,4,5,6,7,8,9. |
| \w |
Matches and letter (either case), number of underscore. |
| \s |
Matches any of the whitespace characters such as space, tab, new line etc. |
| . |
Matches any character except a new line. |
In addition to these groupings we can negate them by adding a preceeding ^ symbol so ^\d means anything that is not a digit. In some cases we may be looking to match a set number of occurances of a pattern, for example for the phone code used in the first example we need to match 5 digits, so we could use :
/\d\d\d\d\d/
But this can very quickly become complex for large or complex patterns, we may also not know exactly how many digits we need to match, for example to validate a percentage value we could have a value of 6, 17 or even 100 percent. This means we are looking for a pattern which has one, two or even three digits, so a straight collection of \d's won't work. Luckily Regular Expressions provide repetition operators that we can use to overcome this problem :
| Regular Expression |
Description |
| {x} |
Matches x occurances of the pattern. |
| {x,y} |
Matches at least x and not more than y occurance of the pattern. |
| {x,} |
Matches at least x occurances of the pattern, with no upper limit. |
| ? |
Matches none or one occurance of the pattern. |
| + |
Matches one or more occurances of the pattern. |
| * |
Matches none of more occurances of the pattern. |
Thus our Regular Expression of percentages becomes :
/\d{1,3}/
If we are looking for a code as our pattern where the first three characters is a combination of the following letters a, b, c, x, y or z followed by three digits. The last section is doable as we can use \d{3}, but we can't use \w as we don't want a d or j in there. Regular Expressions also allows us to specify a custom range between square brackets - [ and ] so we can do the following :
/[a,b,c,x,y,z]{3}\d{3}/
We can use a range operator to simplify the previous pattern, so we could have used :
/[a-c,x-z]{3}\d{3}/
There are a few other groupings which can be used to ensure we get the pattern we are looking for and only the pattern we are looking for, the common ones are :
| Regular Expression |
Description |
| /^ |
Start of the string or line. |
| $/ |
End of the string or line. |
| /g |
Go though the entire string, don't stop on the fist match |
| \<special character> |
Matches a special character such as newline (\n), tab (\t) or a character that is part of the syntax (\{ looks for a { in the pattern). |
| \i |
Case insensitive. |
| \b |
Start or end of a word, allows you to only match on complete words such as cat and not cattle. |
So we can now use these to produce complex Regular Expressions, for example :
| Regular Expression |
Description |
| /^\d{3}[P]$/ |
the string must be 3 digits then capital P. |
| /^\d{3}[P]$/i |
the string must be 3 digits, then a P in either case. |
| /Java/gi |
Matches every occurance of Java in the string in any format of case. |
| /Java\b/gi |
Matches every occurance of Java in the string in any format of case, but will not match Javascript or Java's etc. |
What happens if our pattern can begin with either XY or AB followed by three digits, or we want to match win or wind but not windows ?
Well, in the first case we can use the OR operator to search for XY or AB, followed by a standard \d{3} to find the rest, in the second one we can use the ? operator on the d so that we match zero or one - although we could equally use {0,1}. We can also use brackets to seperate out the sections or our Regular Expression. Therefore our Regular Expressions are :
/^(([X][Y])|([A][B]))\d{3}$/
and
/^win(d)?$/
Now that you have some idea of how RegEx's are created, its time to test your knowledge in the next section to see how much you have learnt.
