Java Regex, match pattern, pair of words

Question

i am using regex to check correctness of the string in my application. I want to check if string has a following pattern: x=y&a=b&... x,y,a,b etc. can be empty.

Example of correct strings:

abc=def&gef=cda&pdf=cdf
=&gef=def
abc=&gef=def
=abc&gef=def

Example of incorrect strings:

abc=def&gef=cda&
abc=def&gef==cda&
abc=defgef=cda&abc=gda

This is my code showing current solution:

    String pattern = "[[a-zA-Z0-9]*[=]{1}[a-zA-Z0-9]*[&]{1}]*";
    if(!Pattern.matches(pattern, s)){
        throw new IllegalArgumentException(s);
    }

This solution is bad because it accepts strings like:

abc=def&gef=def&

Can anyone help me with correct pattern?

Answer 1

You may use the following regex:

^[a-zA-Z0-9]*=[a-zA-Z0-9]*(?:&[a-zA-Z0-9]*=[a-zA-Z0-9]*)*$

See the regex demo

When used with matches() , the ^ and $ anchors may be omitted.

Details :

• ^ - start of string
• [a-zA-Z0-9]* - 0+ alphanumeric chars (may be replaced with \\p{Alnum} )
• = - a = symbol
• [a-zA-Z0-9]* - 0+ alphanumeric chars
• = - a = symbol
• (?: - start of a non-capturing group matching sequences of...
  • & - a & symbol
  • [a-zA-Z0-9]*=[a-zA-Z0-9]* - same as above
• )* - ... zero or more occurrences
• $ - end of string

NOTE : If you want to make the pattern more generic, you may match any char other than = and & with a [^&=] pattern that would replace a more restrictive [a-zA-Z0-9] pattern:

^[^=&]*=[^=&]*(?:&[^=&]*=[^=&]*)*$

See this regex demo

Answer 2

Here you go:

^\w*=\w*(?:&(?:\w*=\w*))*$

• ^ is the starting anchor
• (\\w*=\\w*) is to represent parameters like abc=def
  • \\w matches a word character [a-zA-Z0-9_]
  • \\w* represents 0 or more characters
• & represents tha actual ampersand literal
• (&(\\w*=\\w*))* matches any subsequents parameters like &b=d etc.
• $ represents the ending anchor

Regex101 Demo

EDIT: Made all groups non-capturing.

Note: As @WiktorStribiżew has pointed out in the comments, \\w will match _ as well, so above regex should be modified to exclude underscores if they are to be avoided in the pattern, ie [A-Za-z0-9]

Answer 3

I believe you want this.

([a-zA-Z0-9]*=[a-zA-Z0-9]*&)*[a-zA-Z0-9]*=[a-zA-Z0-9]*

This matches any number of repetitions like x=y , with a & after each one; followed by one repetition like x=y without the following & .