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From: ram@zedat.fu-berlin.de (Stefan Ram)
Newsgroups: comp.lang.python,alt.folklore.computers
Subject: Re: Pythonic way to combine regular expressions?
Date: 25 Sep 2026 19:08:54 GMT
Organization: Stefan Ram
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Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> wrote or quoted:
>What is a good way to combine regular expressions in Python?  We are
>cogitating the most /Pythonic/ way to make this happen.
>    number:
>        sign? integer-constant
>        | sign? floating-point-constant
>    integer-constant:
>        digit-sequence
>    floating-point-constant:
>        fractional-constant exponent?
>        | digit-sequence exponent
>    fractional-constant:
>        digit-sequence? "." digit-sequence
>        | digit-sequence "."
>    exponent:
>        ( "e" | "E" ) sign? digit-sequence
>    sign:
>        "+" | "-"
>    digit-sequence:
>        digit
>        | digit digit-sequence
>    digit:
>        "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"

  Maybe something like this?

import re
from typing import Final

DIGIT: Final[ str ]= r"[0-9]"

DIGIT_SEQUENCE: Final[ str ]= rf"{DIGIT}+"

SIGN: Final[ str ]= r"[+-]"

EXPONENT: Final[ str ]= rf"[eE]{SIGN}?{DIGIT_SEQUENCE}"

FRACTIONAL_CONSTANT: Final[ str ]= \
rf"(?:(?:{DIGIT_SEQUENCE})?\.{DIGIT_SEQUENCE}|{DIGIT_SEQUENCE}\.)"

INTEGER_CONSTANT: Final[ str ]= DIGIT_SEQUENCE

FLOATING_POINT_CONSTANT: Final[ str ]= \
rf"(?:{FRACTIONAL_CONSTANT}(?:{EXPONENT})?|{DIGIT_SEQUENCE}{EXPONENT})"

NUMBER_PATTERN: Final[ str ]= \
rf"^{SIGN}?(?:{INTEGER_CONSTANT}|{FLOATING_POINT_CONSTANT})$"

NUMBER_VALIDATOR: Final[ re.Pattern[ str ]] = \
re.compile( NUMBER_PATTERN )

example = "123"
is_ok = bool( NUMBER_VALIDATOR.match( example ))
print( is_ok )

  (Actually, the Pythonic pattern uses "(" where I wrote "\".)

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