Question

I’ve trying to get a regular expression to work with no luck. I’ve been able to to limit the expression to an alphanumeric number with 10 digits:

(^[a-zA-Z0-9]{10}+$)

however i am also trying to get it allow the $ character with only 1 match in any position.

it should come up as true for something like, pQp3b8ar$8 or k7DdRoB$5W.

Answer 1

Three general notes:

^[a-zA-Z0-9$]{10}$

• the parentheses are not necessary
• {10}+ does not make much sense, drop the plus (there’s no need for a possessive quantifier on a fixed count)
• if you want to allow a dollar sign, just add it to the character class

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To allow a dollar sign only once, you can use an extended version of the above:

^(?=[^$]*\$[^$]*$)[a-zA-Z0-9$]{10}$

The (?=[^$]*\$[^$]*$) is a look-ahead that reads

(?=        # start look-ahead
  [^$]*    #   any number of non-dollar signs
  \$       #   a dollar sign
  [^$]*    #   any number of non-dollar signs
  $        #   the end of the string
)          # end of look-ahead

It allows any characters on the string but the dollar only once.

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Another variant would be to use two look-aheads, like this:

^(?=[^$]*\$[^$]*$)(?=[a-zA-Z0-9$]{10}$).*

Here you can use the .* to match the remainder of the string since the two conditions are checked by the look-aheads. This approach is useful for password complexity checks, for example.