Python regex only check a substring if it follwed by a substring

Question

I want to match all the numbers in a string given in scientific notation, heres my program

import re
txt = '2310163 -204.1154263 -204.1159668 -204.1110188 -204E-9668 200-100'
print re.findall('([+-]?\d+\.?[eE]?[+-]?\d*)', txt)
#                              ^    ^
#                             ex   sg
# allow sg only if its followed by ex

Now the 200-100 is not a valid number but the regex matches it, because I have given a [+-] for the exponent part. Now how to make regex so that it only checks for [+-] if it just followed by [eE] like the number -204E-9668 ?

CertainPerformance · Accepted Answer · 2019-07-23 07:57:53Z

1

Put the whole scientific notation part into an optional group, while matching the possible decimal part separately, before:

[+-]?\d+(?:\.\d+)?(?:[eE][+-]\d+)?
#        ^^^^^^^^^ optional decimals
#                 ^^^^^^^^^^^^^^^^ optional scientific notation

If you want none of the 200-100 part to match because the 200 is right next to the 100, then at the beginning, lookbehind for a space or the beginning of the string, and at the end, lookahead for a space or the end of the string:

(?:(?<=^)|(?<= ))[+-]?\d+(?:\.\d+)?(?:[eE][+-]\d+)?(?= |$)

https://regex101.com/r/SdA295/1

answered Jul 23, 2019 at 7:57

CertainPerformance

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4 Comments

Eular Over a year ago

the end should be (?= |$|\n) for working with python multiline string ?

CertainPerformance Over a year ago

Sure, or with \r\n as well, depending on where the input comes from

Eular Over a year ago

also the start should be (?<=^)|(?<=\s) there can be a tab before the number, i guess?

CertainPerformance Over a year ago

Sure, if that's what your input is like, you can also lookahead for just $ or \s

Oref · Accepted Answer · 2019-07-23 07:59:15Z

1

I think what you're looking for is a positive lookahead

(?=foo)

Here's a good resource on the topic: https://www.rexegg.com/regex-lookarounds.html

answered Jul 23, 2019 at 7:59

Oref

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Toto Over a year ago

What question are you answering?

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