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Suppose I have the following regular expression in Python and I would like to use a variable instead of [1-12]. For example, my variable is currentMonth = 9

How can I plug currentMonth into the regular expression?

r"(?P<speaker>[A-Za-z\s.]+): (?P<month>[1-12])"
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2 Answers 2

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Use string formating to insert currentMonth into the regex pattern:

r"(?P<speaker>[A-Za-z\s.]+): (?P<month>{m:d})".format(m=currentMonth)

By the way, (?P<month>[1-12]) probably does not do what you expect. The regex [1-12] matches 1 or 2 only. If you wanted to match one through twelve, you'd need (?P<month>12|11|10|[1-9]).

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11 Comments

I'd use {m:d} to leave no chance for strings that might break the regex in weird ways to be passed.
@Rosh Oxymoron: Hm, nice idea.
@unutbu (?P<month>[2-9]|1[012]?) better imho
@eyquem: Just curious -- why do you consider it better? It's less readable and also slower for even moderate-sized strings.
@unutbu Because, afaiu and shortly, with (?P<month>12|11|10|[1-9]) the regex motor will have to perform 4 tests to identify a digit 1 to 9 and 1 or 3 tests to identify one of the numbers 12-11-10, while with (?P<month>[2-9]|1[012]?) it will perform only one test to identify a digit 2 to 9 and 3 tests to identify a number 1 or 10 or 11 or 12. But I know that regex motors are optimized, and I'm maybe wrong
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I dont know what you're searching through so I can't test this but try:

(r"(?P<speaker>[A-Za-z\s.]+): (?P<month>%r)" % currentMonth, foo)

where foo is the string you're using the expression on.

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