How to search and replace content programmatically?

You have 2 main options:

• Use search/replace in a script. For example something like the following would replace the word welcome with "bienvenue" everywhere in the wiki:
  {{velocity}}
  #set ($xwql = "where doc.content like '%welcome%'")
  #foreach ($item in $services.query.xwql($xwql).execute())
    #if ($request.confirm == "1")
      #set ($itemDoc = $xwiki.getDocument($item))
      $itemDoc.setContent($itemDoc.getContent().replaceAll(
       "welcome", "bienvenue"))
      $itemDoc.save("Replaced bienvenue")
     * [[$item>>$item]] replaced!
    #else
     * [[$item>>$item]]
    #end
  #end
  
  [[Replace "welcome" by "bienvenue">>?confirm=1]]
  {{/velocity}}
• Use the XWiki Syntax AST to perform more surgical replaces. For example to modify all links so that they are displayed in italics you could use something like:
  {{groovy}}
  import org.xwiki.rendering.block.*
  import org.xwiki.rendering.block.match.*
  import org.xwiki.rendering.listener.*
  
  def mydoc = xwiki.getDocument('Sandbox.VMA')
  def xdom = mydoc.getXDOM()
  
  for (Block block : xdom.getBlocks(new ClassBlockMatcher(LinkBlock.class), Block.Axes.DESCENDANT)) {
    Block parentBlock = block.getParent();
    Block newBlock = new FormatBlock(Collections.<Block>singletonList(block), Format.ITALIC);
    parentBlock.replaceChild(newBlock, block);
  }
  
  // Save the modified XDOM
  mydoc.setContent(xdom)
  mydoc.save('made links in italics')
  {{/groovy}}

  See the Rendering module for more details.