Regular expression to match string containing character - regex

Question

I have a file that contains several strings bounded by single quotations ('). These strings can contain whitespace and sometimes occur over multiple lines; however, no string contains a quotation (') mark. I'd like to create a regex that finds strings containing the character "$". The regex I had in mind: '[^']*\$[^']* can't search over multiple lines. How can I get it to do so?

Answer 1

Your regex can search over multiple lines. If it doesn't there is a mistake in your code outside of it. (hint: [^'] does include newlines).
How about this expression (it prevents useless backtracking):
'([^'$]*\$[^']*)'

Answer 2

You are not telling us which language you are using, so we are left to speculate. There are two issues here, really:
Many regex engines only process one line at a time by default
Some regex engines cannot process more than one line at a time
If you are in the former group, we can help you. But the problem isn't with the regex, so much as it's with how you are applying it. (But I added the missing closing single quote to your regex, below, and the negation to prevent backtracking as suggested in Tomalak's answer.)
In Python 2.x:
# doesn't work
with open('file', 'r') as f:
for line in f:
# This is broken because it examines a single line of input
print "match" if re.search(r"'[^'$]*\$[^']*'", line)
# works
s = ''
with open('file', 'r') as f:
for line in f:
s += line
# We have collected all the input lines. Now examine them.
print "match" if re.search(r"'[^'$]*\$[^']*'", s)
(That is not the idiomatic, efficient, correct way to read in an entire file in Python. I'm using clumsy code to make the difference obvious.)
Now, more idiomatically, what you want could be
perl -0777 -ne 'while (m/\x27[^\x27$]*\$[^\x27]*\x27/g) { print "$&\n" }' file
(the \x27 is a convenience so I can put the whole script in single quotes for the shell, and not strictly necessary if you write your Perl program in a file), or
#!/usr/bin/env python
import re
with open('file', 'r') as f:
for match in re.match(r"'[^'$]*\$[^']*'", f.read()):
print match
Similar logic can be applied in basically any scripting language with a regex engine, including sed. If you are using grep or some other simple, low-level regex tool, there isn't really anything you can do to make it examine more than one line at a time (but some clever workarounds are possible, or you could simply switch to a different tool -- pcregrep comes to mind as a common replacement for grep).
If you have really large input files, reading it all into memory at once may not be a good idea; perhaps you can devise a way to read only as much as necessary to perform a single match at a time. But that already goes beyond this simple answer.