5
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I'm learning regex. Here I'm checking a string of type aA1 but want also to check it is only 3 characters long. Is there a better way of doing it please?

str = raw_input("input in form 'aA1' please >")
check = re.compile("[a-z][A-Z][0-9]")
if len(str) == 3:
    validate = check.match(str)
    if validate:
        print "Correct string"
    else:
        print "Incorrect string"
else:
    print "Wrong length"
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  • \$\begingroup\$ (Of course there should be 'import re' at the top \$\endgroup\$ – j-o Oct 5 '14 at 10:55
5
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Ref. http://www.regular-expressions.info/anchors.html

Anchors are a different breed. They do not match any character at all. Instead, they match a position before, after, or between characters. They can be used to "anchor" the regex match at a certain position. The caret ^ matches the position before the first character in the string. Applying ^a to abc matches a. ^b does not match abc at all, because the b cannot be matched right after the start of the string, matched by ^. See below for the inside view of the regex engine.

Similarly, $ matches right after the last character in the string. c$ matches c in abc, while a$ does not match at all.

With anchors you may skip length test entirely.

So:

import re
str = raw_input("input in form 'aA1' please >")
check = re.compile("^[a-z][A-Z][0-9]$")
validate = check.match(str)
if validate:
    print "Correct string"
else:
    print "Incorrect string"
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  • \$\begingroup\$ In fact a caret isn't needed with re.match at the beginning of the expression in line 3 because .match only matches a pattern occurring at the beginning of a string. re.search would require the opening caret. \$\endgroup\$ – j-o Oct 6 '14 at 15:25

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