replace with regular expression python

Question

I have numeric strings in fortran format that look like this 0.1245-102 float conversion doesn't work because format is not expected : 0.1245e-102

I use this command

re.sub(r"[0-9]-[0-9]",r"e-","0.1245-102")

I 'm very new to regular expression and with this way i obtain 0.124e-02 how can i keep 5e-1 in new result ?

@ANSWERERS -- Other ways which these numbers could possibly appear (I think): 1-10, 1.-10, -1-10 -- I think the accepted solution should handle these just in case ... — mgilson
– mgilson, Commented Apr 4, 2013 at 15:04

Martijn Pieters · Accepted Answer · 2013-04-04 16:15:57Z

3

You are looking for:

r'(?<=\d|\.)-(?=\d)'

which matches a - if there are digits or a dot) before, and digits after.

Demo:

>>> exponent = re.compile(r'(?<=\d|\.)-(?=\d)')
>>> exponent.sub('e-', '0.1245-102')
'0.1245e-102'
>>> float(exponent.sub('e-', '0.1245-102'))
1.245e-103
>>> float(exponent.sub('e-', '1-10'))
1e-10
>>> float(exponent.sub('e-', '1.-10'))
1e-10
>>> float(exponent.sub('e-', '-1-10'))
-1e-10

To handle both - and D+ (negative and positive exponents), you could use:

r'(?<=\d|\.)D?(?=(?:\+|-)\d)

which would allow for and replace an optional D, leaving the - or + in place when replacing:

>>> exponent = re.compile(r'(?<=\d|\.)D?(?=(?:\+|-)\d)')
>>> float(exponent.sub('e', '0.1245-102'))
1.245e-103
>>> float(exponent.sub('e', '456D+5'))
45600000.0

This does allow for the D to also precede the -, so 0.1245D-102 would be valid too, but it simplifies the replacement handling.

edited Apr 4, 2013 at 16:15

answered Apr 4, 2013 at 15:08

Martijn Pieters

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6 Comments

froggy Over a year ago

yes it sounds like this . I forgot positives exponent look like this "456D+5"

froggy Over a year ago

yes but it fails for positif have i to include this call in a try - except test ? or is there other thing to do with compile function ?

Martijn Pieters Over a year ago

What do your positive exponents look like? You can do a try: except: or first test if there is a match.

froggy Over a year ago

positives are for example "456D+5"

Martijn Pieters Over a year ago

@froggy: So D+ needs to be replaced with e?

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