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    <p>Well, you could make use of the fact that Python's regex engine only allows fixed-length regular expressions in lookbehind assertions:</p> <pre><code>import re regexes = [r".x{2}(abc|def)", # fixed r"a|bc", # variable/finite r"(.)\1", # fixed r".{0,3}", # variable/finite r".*"] # variable/infinite for regex in regexes: try: r = re.compile("(?&lt;=" + regex + ")") except: print("Not fixed length: {}".format(regex)) else: print("Fixed length: {}".format(regex)) </code></pre> <p>will output</p> <pre><code>Fixed length: .x{2}(abc|def) Not fixed length: a|bc Fixed length: (.)\1 Not fixed length: .{0,3} Not fixed length: .* </code></pre> <p>I'm assuming that the regex itself is valid.</p> <p>Now, how does Python know whether the regex is fixed-length or not? Just read the source - in <code>sre_parse.py</code>, there is a method called <code>getwidth()</code> that returns a tuple consisting of the lowest and the highest possible length, and if these are not equal in a lookbehind assertion, <code>re.compile()</code> will raise an error. The <code>getwidth()</code> method walks through the regex recursively:</p> <pre><code>def getwidth(self): # determine the width (min, max) for this subpattern if self.width: return self.width lo = hi = 0 UNITCODES = (ANY, RANGE, IN, LITERAL, NOT_LITERAL, CATEGORY) REPEATCODES = (MIN_REPEAT, MAX_REPEAT) for op, av in self.data: if op is BRANCH: i = sys.maxsize j = 0 for av in av[1]: l, h = av.getwidth() i = min(i, l) j = max(j, h) lo = lo + i hi = hi + j elif op is CALL: i, j = av.getwidth() lo = lo + i hi = hi + j elif op is SUBPATTERN: i, j = av[1].getwidth() lo = lo + i hi = hi + j elif op in REPEATCODES: i, j = av[2].getwidth() lo = lo + int(i) * av[0] hi = hi + int(j) * av[1] elif op in UNITCODES: lo = lo + 1 hi = hi + 1 elif op == SUCCESS: break self.width = int(min(lo, sys.maxsize)), int(min(hi, sys.maxsize)) return self.width </code></pre>
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    1. POdetermine if regular expression only matches fixed-length strings
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    1. USTim Pietzcker
    1. USTim Pietzcker
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    1. COI am coding in C, using the PCRE library... Thanks anyway :-)
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      1. USPaul Praet
 

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