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Editorial Team
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Asked: 2026-06-08T21:26:49+00:00

I’m trying to clean out some code by removing either leading or trailing white

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I’m trying to clean out some code by removing either leading or trailing white space characters using PyParsing. Removing leading white spaces was quite easy as I could make use of the FollowedBy subclass which matches a string but does not include it. Now I would need the same for something that follows my identifying string.

Here a small example:

from pyparsing import *

insource = """
annotation (Documentation(info="  
  <html>  
<b>FOO</b>
</html>  
 "));
"""
# Working replacement:
HTMLStartref = OneOrMore(White(' \t\n')) + (FollowedBy(CaselessLiteral('<html>')))

## Not working because of non-existing "LeadBy" 
# HTMLEndref = LeadBy(CaselessLiteral('</html>')) + OneOrMore(White(' \t\n')) + FollowedBy('"')

out = Suppress(HTMLStartref).transformString(insource)
out2 = Suppress(HTMLEndref).transformString(out)

As output one gets:

>>> print out
annotation (Documentation(info="<html>
<b>FOO</b>
</html>
 "));

and should get:

>>> print out2
annotation (Documentation(info="<html>
<b>FOO</b>
</html>"));

I looked at the documentation but could not find a “LeadBy” equivalent to FollowedBy or a way how to achieve that.

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1 Answer

  1. Editorial Team

    What you are asking for is something like “lookbehind”, that is, match only if something is preceded by a particular pattern. I don’t really have an explicit class for that at the moment, but for what you want to do, you can still transform left-to-right, and just leave in the leading part, and not suppress it, just suppress the whitespace.

    Here are a couple of ways to address your problem:

    # define expressions to match leading and trailing
# html tags, and just suppress the leading or trailing whitespace
opener = White().suppress() + Literal("<html>")
closer = Literal("</html>") + White().suppress()

# define a single expression to match either opener
# or closer - have to add leaveWhitespace() call so that
# we catch the leading whitespace in opener
either = opener|closer
either.leaveWhitespace()

print either.transformString(insource) 


# alternative, if you know what the tag will look like:
# match 'info=<some double quoted string>', and use a parse
# action to extract the contents within the quoted string,
# call strip() to remove leading and trailing whitespace,
# and then restore the original '"' characters (which are
# auto-stripped by the QuotedString class by default)
infovalue = QuotedString('"', multiline=True)
infovalue.setParseAction(lambda t: '"' + t[0].strip() + '"')
infoattr = "info=" + infovalue

print infoattr.transformString(insource)
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