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It's a simple question about regular expressions, but I'm not finding the answer.
I want to determine whether a number appears in sequence exactly two or four times. What syntax can I use?
\d{what goes here?}
I tried \d{2,4}, but this expression accepts three digits as well.
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Add the $ anchor. /^SW\d{4}$/ . It's because of the \w+ where \w+ match one or more alphanumeric characters. \w+ matches digits as well.
Occurrence Indicators (or Repetition Operators): +: one or more ( 1+ ), e.g., [0-9]+ matches one or more digits such as '123' , '000' . *: zero or more ( 0+ ), e.g., [0-9]* matches zero or more digits. It accepts all those in [0-9]+ plus the empty string.
To match any number from 0 to 9 we use \d in regex. It will match any single digit number from 0 to 9. \d means [0-9] or match any number from 0 to 9. Instead of writing 0123456789 the shorthand version is [0-9] where [] is used for character range.
Regular expressions, called regexes for short, are descriptions for a pattern of text. For example, a \d in a regex stands for a digit character — that is, any single numeral 0 to 9. Following regex is used in Python to match a string of three numbers, a hyphen, three more numbers, another hyphen, and four numbers.
There's no specific syntax for that, but there are lots of ways to do it:
(?:\d{4}|\d{2}) <-- alternation: four digits if possible, else just two \d{2}(?:\d{2})? <-- two digits, plus two more if possible (?:\d{2}){1,2} <-- two digits, times one or two So, for example, to match strings consisting of one or more letters A–Z followed by either two or four digits, you might write ^[A-Z]+(?:\d{4}|\d{2})$; and to match a comma-separated list of two-or-four-digit numbers, you might write ^((?:\d{4},|\d{2},)*(?:\d{4}|\d{2})$ or ^(?:\d{2}(?:\d{2})?,)*\d{2}(?:\d{2})$.
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