Regular Expressions Basics
Repetitions
Repetitions simplify using the same pattern several consecutive times. They also allow for flexible length searches, so you can match 'aaZ' and 'aaaaaaaaaaaaaaaaZ' with the same pattern.
{ } Ranges
You can use the following syntax for defined ranges:
| Pattern | Description |
|---|---|
{n} | Repeat the previous symbol exactly n times |
{n,} | Repeat the previous symbol n or more times |
{min,max} | Repeat the previous symbol between min and max times, both included |
So a{6} is the same as aaaaaa, and [a-z]{1,3} will match any text that has between 1 and 3 consecutive letters.
Note: In repetitions, each symbol match is independent. If
[a-z]{1,3}first matches with 'a', on the next letter it can match with anything in the [a-z] range, not only 'a'.
Other Ranges
You can use the following syntax for other types of ranges:
| Pattern | Description |
|---|---|
* | Repeat the previous symbol 0 or more times |
+ | Repeat the previous symbol 1 or more times |
? | Repeat the previous symbol 0 or 1 times |
Note:
*is the same as{0,},+is the same as{1,}, and?is the same as{0,1}
A common use for ? is to allow both singular and plural words: cats? will match either cat or cats.
Repetitions are greedy on searches; they try to get the largest match possible. Sometimes that's undesired, so you can force a lazy search by adding ? after * or +.
The ? instructs the regex engine to make a lazy search, which gives the smallest match possible.
Greedy Search: a.*a will find
Lazy Search:
a.*?a will find
Note: Regex is not recommended for parsing XML or HTML. See: http://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags However it can find what you need for simpler things.
In the next lesson, you'll learn about Alternations.