Regular expression with if condition

Question

I am new to regular expression. I am trying to construct a regular expression that first three characters must be alphabets and then the rest of the string could be any character. If the part of the string after first three characters contains & then this part should start and end with ".

I was able to construct ^[a-z]{3}, but stuck at conditional statement.

For example abcENT and abc"E&T" are valid strings but not abcE&T.

Can this be done in a single expression?

Answer 1

In most regex flavors, you may use simple lookaheads to make sure some text is present or not somewhere to the right of the current locations, and using an alternation operator | it possible to check for alternatives.

So, we basically have 2 alternatives: there is a & somewhere in the string after the first 3 alphabets, or not. Thus, we can use

^[A-Za-z]{3}(?:(?=.*&)".*"|(?!.*&).*)$

See the regex demo

Details:

• ^ - start of string
• [A-Za-z]{3} - 3 alphabets
• (?:(?=.*&)".*"|(?!.*&).*) - Either of the two alternatives:
  • (?=.*&)".*" - if there is a & somewhere in the string ((?=.*&)) match ", then any 0+ characters, and then "
  • | - or
  • (?!.*&).* - if there is no & ((?!.*&)) in the string, just match any 0+ chars up to the...
• $ - end of string.

In PCRE, or .NET, or some other regex flavors, you have access to the conditional construct. Here is a PCRE demo:

^[A-Za-z]{3}(?(?=.*&)".*"|.*)$
            ^^^^^^^^^^^^^^^^^

The (?(?=.*&)".*"|.*) means:

• (?(?=.*&) - if there is a & after any 0+ characters...
• ".*" - match "anything here"-like strings
• | - or, if there is no &
• .* - match any 0+ chars from the current position (i.e. after the first 3 alphabets).

Answer 2

A conditional statement could be use with | and groups, but it probably will be complicated.

^[a-z]{3}([^&]*$|".*"$)

You might think about using plain old string manipulation for this task, it probably will be simple

Answer 3

Yeah this is possible, it is not really an if, but in your case you can make an "or" with regex capturing Group. Your regex would look something like that:

\d{3}(\".*\"|[^&]*)

P.S. here is a good site to test and learn These things: https://regex101.com/