I am creating an ANT script for which I am using a fileset and a regex to get my file and to get an expression in that file.
I also need to store the expression in a spreadsheet, which I am not sure how to do it.
I have learned somewhere that I could use replaceregexp, but I am not sure how do I do that.

I have used target tasks to do the search for the file and expression.

ANT is not a programming language so this is not a trivial problem to solve. The following example uses:

groovy task implement the parsing logic
Apache POI to write the excel file
Apache ivy to manage the 3rd party jar dependencies.

Example

This is example counts the number of occurrences of the string "one" in the resources sub directory:

├── build.xml
├── resources
│   ├── file1.txt
│   ├── file2.txt
│   └── file3.txt
└── results.xlsx

Results are also written to an excel file "results.xlsx"

parse-files:
   [groovy] File:file1.txt Count:1
   [groovy] File:file2.txt Count:0
   [groovy] File:file3.txt Count:1

build.xml

<project name="demo" default="parse-files" xmlns:ivy="antlib:org.apache.ivy.ant">

  <available classname="org.apache.ivy.Main" property="ivy.installed"/> 

  <target name="parse-files" depends="init" >
    <taskdef name="groovy" classname="org.codehaus.groovy.ant.Groovy" classpathref="groovy.path"/>

    <fileset dir="resources" id="filesToParse" includes="*.txt"/>

    <groovy>
      import org.apache.poi.ss.usermodel.Workbook
      import org.apache.poi.xssf.usermodel.XSSFWorkbook
      import org.apache.poi.ss.usermodel.CreationHelper
      import org.apache.poi.ss.usermodel.Sheet
      import org.apache.poi.ss.usermodel.Row

      Workbook wb = new XSSFWorkbook();
      Sheet sheet = wb.createSheet("Regexp results");
      CreationHelper helper = wb.getCreationHelper();
      rowpos = 0

      project.references.filesToParse.each {
        def file    = it.file
        def content = file.text

        // Search file
        def findOneStr = content =~ /one/

        println "File:${file.name} Count:${findOneStr.count}"

        // Save to Excel row
        Row row = sheet.createRow(rowpos++)
        row.createCell(0).setCellValue(helper.createRichTextString(file.name));
        row.createCell(1).setCellValue(findOneStr.count);
      }

      // Save excel file
      new File("results.xlsx").withOutputStream {
        wb.write(it)
      }
    </groovy>
  </target>

  <target name="init" depends="install-ivy" description="Resolve dependencies">
    <ivy:cachepath pathid="groovy.path">
      <dependency org="org.codehaus.groovy" name="groovy-all" rev="2.4.3" conf="default"/>
      <dependency org="org.apache.poi" name="poi-ooxml" rev="3.10.1" conf="default"/>
    </ivy:cachepath> 
  </target>

  <target name="install-ivy" description="Install ivy" unless="ivy.installed">
    <mkdir dir="${user.home}/.ant/lib"/>
    <get dest="${user.home}/.ant/lib/ivy.jar" src="http://search.maven.org/remotecontent?filepath=org/apache/ivy/ivy/2.4.0/ivy-2.4.0.jar"/>

    <fail message="Ivy has been installed. Run the build again"/>
  </target>

</project>