Regular Expressions are a always tricky and sometimes they stir the mind. I was constructing a regular expression for following conditions:
- String should min of 3 characters and max of 6 characters
- First three characters can be [a-zA-Z]
- Either of 4,5 or 6th character should be [0-9]
- Remaining characters can be [a-zA-Z] but not [0-9]
This query troubled me like anything. I tried so many combinations but could not make it. The main problem was making sure that there is one number in the 4,5 or 6th position and limiting it to a max of 6 and min of 4 characters in length.
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So finally the RegEx is as follows:
^(?=.*[0-9])(?!.*[0-9].*[0-9])(?!.*[0-9].*[0-9].*[0-9])[a-zA-Z]{3}[a-zA-Z0-9]{1,3}$
The above regex works for the specified condions and mes sure that there is only one digit in the string.
Following is an analysis of the same:
The RegEx two blocks: first block checks whether a certain pattern exists in the whole string and second block checks for [a-zA-Z] three times and [a-zA-Z0-9] one to three times.
^ and $ make sure that the pattern is verified from beginning to the end of the given string.
The first block:
(?=.*[0-9])(?!.*[0-9].*[0-9])(?!.*[0-9].*[0-9].*[0-9])
The ?= looks ahead in the whole string whether there exists any character of [0-9]. The ?! looks ahead in the whole string whether there is not two or three [0-9]. That means this block makes sure that the whole string contains only one digit [0-9] and not more than that. But it will not verify where exactly the [0-9] should come. It will be taken care by the second block.
The second block:
[a-zA-Z]{3}[a-zA-Z0-9]{1,3}
In the second block, first part verifies whether the first three characters are [a-zA-Z] once it is satisfied, second part checks the next 1-3 characters contain [a-zA-Z0-9].
Isn’t that a wonderful RegEx.. 
You can visit this link for a quick reference on Regular Expressions.
If you have any regular expression that is eating your head then probably this forum can solve it.. give it a try.
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